I've fitted R1 fuel injection throttle bodies to my A15. They are well oversized but FI is a bit more forgiving, just means the low end throttle response will be a bit more intense (not tuned or plumbed up yet but getting there). From memory they're around 45mm ID and the rubber mounts taper down to line up quite nicely with 1-1/2" exhaust pipe (OD, wall thickness is 1.6mm).

If you're not into fuel injction/ ECU etc. and only mild boost I'd support most of the other comments that say go for a single carb, draw through setup. Bolt it all onto the standard manifolds. The more stuff you change the longer it'll take to get it going and the more problems you'll come across.

What ever you do, have fun with it!