Sorry about coming late to the party, but wanted to contribute my 904 story in case you have not committed to a trans yet. I got a NEW 904 (actually a newer 1976 version) core from a friend and had it built for my 408 stroker combo. It had the 2.76 low gear set, and a modified valve body, still forward pattern, with down shift, built by Chuck Lofgren. Full throttle shifts were set at 5800 and it worked like a dream behind that 408 with 518 tq at 4500 rpm, and 503 hp at 5700. A ten inch converter from A&A stalled to 4,000 rpm, yet would cruise at 2900 rpm at 70 mph with 3.55 gears out back . A converter like that one is as loose as i would ever use for a car weighing 3400 lbs that i want to get on the highway with. The newer version of the 904, I believe about 1975 and up, can be built pretty strong. The guts are used in Superstock
426 Hemi cars.
The smaller trans is worth about 20 hp? Plus you get a wide ratio set, 276,154, 1 final so you don't have ro run as deep a gear to get a reasonable 60 ft time.
