I have a '71 with a 454 complete with new badges. The engine is snug for fit, but no dents in the firewall, or cuts to the frame. My stock exhaust manifold might have helped some for the frame. Stock engine mount locations were used as well. I am running 3:07's with a sm465, with 35's, and really like the combo. 60 miles an hour @ 2000 rpm. It is like overdrive without the OD. The 454 has plenty of torque on the bottom to over come the tires and gear. No replacement for more displacement.
http://67-72chevytrucks.com/vboard/a...1&d=1192915029