Or you can keep your 904 and put a Gear Vendors behind it for a lot less $, hassle and fabrication.
(ding up the tunnel a bit, add wiring and a switch and shorten the driveshaft)
They are still not cheap, but cheaper than the tremec kit, no pedal swap, your speedometer gear and cable still function etc. etc.
I put T5's behind my slant cars, but I buy the cars with the 3rd pedal already installed. The T5 might be weak behind your engine combo, and there is a lot of tunnel and floor fab to make it work.