904: should I switch to 727?

If you are gonna spend money on something like a valve body scrap the 904 and go 727 instead. You'll be money ahead

same valve body

904 don't detonate is my primary reason to not run a 727 behind a SB. You can spank a 727 sprag spinning the tires and them suddenly hooking in first gear. Only way I'll ever run a 727 is with a billet drum in it and that isn't even a guarantee to not explode (seen two of those let loose).