You'll need to look at your VIN tag (under the hood, square aluminum tag) and there's the axle code on it. It will read "T342" or something like that; that's the first thing you need to know. As for IRS, it's heavier than the live axle you currently have. You could weld in mounts for it, but it probably wouldn't be worth the hassle.
Ah... it would have been good to mention that in your first post, but that's okay. If you are intent on using the IRS, it has a 7.5" crown gear vs. your current 6.7" gear, and the chances of finding an LSD IRS are pretty good. Cressida sedans came with the same IRS and LSD as well (but not all of them have the lsd), so that should expand your parts availability a bit.
There are only 4 bolts that hold the entire IRS sub-frame to the car; the only fabbing you'd need to do would be provision for shock towers (obviously) and reinforced mounting pads for the IRS to bolt to.
In that case, you'd probably want one out of an '87+ Supra turbo. I really couldn't tell you if there's a difference between turbo and non-turbo diffs, but you could always search the "supra" section of TN for more info.
The T series diff (6.7") in your 3rd gen will not live with 500HP.
If you're willing to stick to 300-350HP then the solution is easy.
The IRS from the RA65 GT-S or the MA61 Supra has an F series diff (7.5") and fits with little trouble.
Cut off your panhard rod bracket, drill 4 holes for the IRS K-frame and bolt it all up.
Those 4 holes must be measured accurately or your car will pull to the side.
GT-S disc brakes are a bonus but require a matching change to the master cylinder (drum brakes use much less pressure than disc, so drum brake cars have pressure restrictors built into the rear circuit).
Wheel stud pattern and track are unaltered.
I've been told that the MX73 Cressida IRS is the same as the MA61 Supra IRS but you would have to verify this yourself.
If the donor car did not have LSD then go to a 4WD shop.
The Hilux (aka pickup, Tacoma, 4Runner) used the F series diff and the 4WD shops have a large range of ratios and LSD for the F series diff.
For 500HP you need the G series diff (8.0").
Luckily these are common under the Land Cruiser (in a large range of ratios and optional LSD) and under the MX83 Cressida (limited ratios and optional LSD).
I've been told that the MX83 IRS is very similar to the MX73 Cressida (and MA61 Supra) but there are small differences.
I'm moderately sure the MX83 IRS will do the job for you but you would have to measure one up to be sure.
4WD shops have a large range of ratios and LSD for the G series diff.
A complete Landcruiser solid axle with a G series diff could be used.
You need to pay a professional to shift the mounts (weld heat distorts the axle tubes but the pro's straighten them up again).
The track is significantly wider than the Celica track but this may be good for you (add Celica GT-S or Supra flares).
The stud pattern is different, so chnage your rear rims or ask the pro shop to change the stud pattern.
4WD shops have a large range of ratios and LSD for the G series diff.
It might be possible to use the RA65 GT-S or MA61 Supra rear end with an MX83 G series diff in the middle.
I'm not sure if the diff housing is the same size.
If it's different then I'm not sure if the half shafts can accommodate the difference.
I'm also not sure if the half shafts will live with power but it shouldn't be too hard to get stronger ones machined up.
Lastly, how on Earth are you getting 500HP.
The 22R will not give that.
Not even in 22R-TE turbo form (which has a stronger block).
A heavily modified 3S-GTE could but then you get all the fun of finding a suitable transmission (in Australia we use the SA61 Corona transmission but you didn't get that in America and it wouldn't be strong enough anyway).
A 7M-GTE or a 2JZ-GTE can make 500HP but they are not an easy fit and the weight distribution is all wrong (cornering becomes lethal).
A modified 1UZ-FE with turbos can make 500HP and fits in the engine bay (I'm doing similar with a 1980 Carina) but it is not a trivial project.
If 500HP is wishful thinking and 300Hp is more realistic then stick to the RA65 or MA61 Supra IRS and keep it simple.
If 500HP is real then go for the MX83 G series diff.
wow ok I need to copy all that down and tag what I need lol, As for the HP goal Im not keeping the 22 in there I'm having one built I just got to find one that has the exhaust on the drivers side before I consider a pass sided exhaust. I knew my lofty project goals arent gonna be easy, easy isnt fun for me:naughty: As for the stocker I'm going to find a small turbo kit that'll work and swap the head out for a better freer flowing one, exhaust from the dp back I'm custom making myself.
Driver's side exhaust?
I'm guessing that Oregon is somewhere in the US, which makes it LHD (I'm RHD in Australia, so the exhaust is on the passenger side).
So I guess you mean pipes exiting just behind the left hand door, possibly as Laker pipes.
By "having one built" do you mean you are getting a 22R built or do you mean you are looking for any engine that will make 500+ HP?
A 22R-TE heavily modified might just be able to make that power but it will be a grenade engine (requiring frequent rebuilds after limited kilometres), will be an absolute pig to drive (no low down torque, very noisy, hard in stop/start traffic, overheating problems, thirsty) and will not pass emissions tests.
I'm looking at a newer 1jz (one that was in last year supra before they went 2jz) or a really early 2jz to build the crap out of; I'm currently searching out engines that make a decent amount of stock HP. Ya Oregon is on the West Coast of the US, as for the pipes I might have them exit right before one of the rear tires but more likely keep it rear exiting, the custom is exhaust is going to be for the mainly stocker 22rec thats sitting it in now, once I get an engine for the swap picked out then I'll be doing semi-custom made:naughty:
The 1JZ and 2JZ will not be an easy fit.
A few people have done it before to early RWD Celicas (all 3 RWD generations have similar size engine bays) and all of them said it was a pain in the butt and that they would never do it again.
The main trouble is sheer length.
Even when placed as far back in the engine bay as possible, the extra 2 cylinders have to sit ahead of the front wheels.
This absolutely kills handling and makes it a death trap on corners.
1. Recess the firewall by at least 3" .
Expensive and difficult but the only correct way to keep the handling with that engien and that body.
The Supra was designed for the similar size 5M-GE 6 cylinder engine.
An upgrade to a 1Zj or 2JZ is dead easy and has been done many times before.
It's not hard for a worked 2JZ-GTE to give you the power you want in this chassis.
And the Supra already has IRS and an F series diff which you can upgrade to a G series diff (see caveats in previous posts).
The Lexus V8 is very compact and fits into the RA64 engine bay.
Add twin turbos, a suitable cam and an aftermarket computer and it will give you the power you want.
The 1UZ-FE is so over engineered that it handles up to about 500-600HP with bolt-ons before it needs any internal work.
You will still want to upgrade the rear end.
Not quite a trivial upgrade but still within reason.
I've been doing something similar to my 1980 Toyota Carina. http://members.iinet.net.au/~stepho/mods.htm#CarinaV8
I'll repeat what I said at the beginning - a 1JZ or 2JZ in an RA64 Celica is very hard (if done right) or a death trap (if done wrong).