Welcome to the forums! First off, don't bother with a high-flow catalytic converter. Just remove the cat completely. It's not a legal requirement here in NZ

. Secondly, FX-GT's with a big bore canon-type muffler are way overdone. In my opinion, a high-flow baffled muffler such as a Flowmaster, Dynomax or Magnaflow is a much better choice. As for the intake, avoid putting a pod filter straight on the end of the MAF sensor. This will only draw hot engine bay heat into your ITB's. Either replace your stock filter with a K&N panel filter, route the pod filter outside of the engine bay to make a cold air intake or run aftermarket trumpets on your ITB's.
I'm not 100% sure about the 16v exhaust manifold fitting the 20v, but I know that the 20v manifold can be made to fit the 16v with a bit of modification to the stud holes, so it might work the other way around too. There was actually a guy who listed an aftermarket 20v exhaust manifold on TradeMe the other day for a really good price. Needless to say, it got snapped up fast and has now been sold. The original auction can be viewed
here.
There are loads of links online in regards to 20v modifications, so I'd suggest having a browse to get all sorts of different ideas. Best of luck, and keep us updated with your progress!