Rear discs are overrated.
Very little gain for the cost.
The standard TA22 rear axle has 15mm wide bearings.
Most rear disc cars have 45mm bearings.
This is becuase discs tend to flex the axle more.
If you aren't braking enough to worry about this then your 9" drums are good enough.
If you do hard braking (eg racing) then you need to also worry about axle flexing and bearing failure.
The method I used on my 77 Celica (same rear axle as your 74) was to use an MA4# or MA6# solid axle from a Supra or Japanese imported Celica XX. This gives you discs, stronger F series diff (easier to find LSD for F series instead of rare T series LSD) and the entire assembly is wider for better cornering. You need a suspension shop to cut the suspension mounts (including the panhard rod mount and spring perches) from your old axle housing and then have them welded to the Supra housing. I can't remember if the F series diff flange bolts to the TA22 tail shaft flange (75% sure it does). You need to alter the brake bias valve so that more pressure goes to the back (drums require only a little pressure, discs require much more).
The stock front brakes should be upgraded.
The 9" rear brake drums are perfectly adequate.
The T series diff might not handle the extra power.
So the complete rear end from a MA4# Supra with F series diff and discs would be a good combination.
If you have wide wheels then you might need to flare the rear wheel arches.
If you have standard size wheels or the offset is not too bad then you don't have to modify them.
I have 15x6-20 Supra rims with 225 tyres, so I had to have a small flare job done to the arches (15mm each side from having the arch lips bent up, plus another 10mm each side for stretching the arch outwards a little).
Since the '74 TA22 had slightly narrower rear chassis rails than my '77 RA28, you may not have to do this but you should measure it anyway.
Personally, I would just leave the TA22 rear end there.
If the T series diff keeps dieing then replace the entire rear end with the MA4# Supra rear end.