My 3550 came with the 2.95 first gear in it. The stock (early) t5 has a 3.27, the world class or t5-z has a 2.95. The only mod I had to do when switching from the t5 to the 3550 was move the trans cross member back 1". I just cut the tack welds on the slide tube at the edges of the crossmember and slid it as far back as possible, this allowed *just* enough clearance. (then re-welded it) If you go with the 3550, it uses the t5 yoke, if you go tremec TKO it uses a c6 yoke. (just fyi)
Oh and Will runs a TKO behind his 45xhp 53xtq @ the wheels turbo cobra and it lived all season on slicks without so much as a whimper... so the ratings they use are probably a little conservative.
Dave-