I have an SC400 if I put a 2JZ in it, can I use a full catback exhaust for Supra exhaust or an SC400 exhaust? As I know kind of a stupid question but it is unclear to me if I could use a Supra exhaust?

 

Supra exhaust is shorter than sc, you'll need to extend it, most people who plan droping 2j in their 400 upgrade the stock exhaust for more power gains from the turbo.
the stock exhaust will be very restrictive, i would custom fab the rear section where it meets the downpipe and goes all the way back to the back 3 or 4" pipe+ Muffy