Properly built the 904 is a excellent transmission,i ran one behind my 520 horse stroker with excellent results,and have the same tranny behind my 400+ h.p. 360 with many many hard 1/4 mile passes..
A 904 will probably be the way I go. Already have what is probably no more than a core. Worked 20 years ago I heard. May take on rebuilding it as well.
Heard some have a short first gear. How do you tell.