I just put a TKO 600 in my 69 GTO. You cannot imagine how much better it drives. The only thing I sacrificed was the porch, so I just bought another one and kept my original. No one would ever know when I change the four speed back. ( in case I sell it or concourse restore it ).
I did put a new body mounts all the way around to gain as much clearance between the body and frame as I could .
I bought it from American powertrain and they shave the top of the five speed to fit. Console works perfectly. P.m. me if you have questions. 2.87 first, 4th is 1:1, 5th is .64 OD.
Crossmember just slides back two inches and the frame already had the holes drilled it from the factory.
Need to trim the transmission mount to fit into the crossmember.
Cannot tell you how worthwhile it is! 75 miles an hour in 2000 RPM. Pontiacs love that RPM range!!
Cool calculator here.
http://www.tremec.com/calculadora.php