don't waste you're time with a 12a, get a 13b. especialy if you're going n/a.
u'll want a series 4 RX7 turbo gearbox with an RX4 bellhousing.
These are a realy strong box and are fairly big but you wont break one.
you shouldn't need to cut you're firewall either. i didn't in my ke55 and you would think a t18 would have to have a bigger tunnel? you will probably need to cut a fair chunk out of you're x/member aswell to get the motor sitting a bit lower and further back.