I don't know what your budget is but you may want to check with J.W. Performance
racewithjw.com 321-632-6205. They may have a bolt on bellhousing so you can use you existing 904. About 10 years ago we took a 518 smallblock trans and removed (cut) the bell off and used a bolt on bell to mate to a 440 big block. It comes as a kit and bolts to the front pump. Worked out for us. That way you know what you got and if your trans is in good working order it may be cheaper in the long run. They make bells(SFI) for many conversions like putting a powerglide behind a mopar block etc. .Ask if you still can use your stock converter.Our conversion we had to use a 10'' converter as the factory 12'' didn't fit the thick new bell. I think you may be ok with the small converter from your 904.It may save you the changing of shifters, driveshaft,beating the flange of the floor. If I remember the cost was around $600 for everything.Just a thought.