904: should I switch to 727?

I bought a used "supposedly built" (to what degree I have no idea) A-904 transmission for my Duster back in 2010 for $100, maybe less. First it was behind a roughly 270 HP (crank HP uncorrected at 5000' elevation) 360 for 25k+ miles, then behind my current 360 HP (same, crank HP at 5000') 360 for another 10k miles or so. Most of the time I had it there was no indication the valve body had been modified with a shift kit or anything like that until a few years ago when my valve body cracked and I installed a later one I modified doing a "poor man's shift kit" type deal with the guidance of some super knowledgeable FABO members. Still shifts perfectly and when I pulled the pan to replace the valve body there was minimal clutch material in the bottom of the pan and the fluid looked perfect.

Keep the 904 and just make sure it has some form of shift kit in it to firm up the shifts and reduce overlap between gears when shifting, should last you a long time. The original 904 my Duster came with behind a worn-out 318 did give up the ghost but that was a BONE stock original 1970 transmission that had never been rebuilt and I beat on it pretty hard doing a lot of manual WOT 1-2 upshifts.

I'd personally only use a 727 behind a big block and/or in a heavy vehicle or truck that does a lot of towing and hauling. They're kinda like the Ford C6 or GM TH-400, beefy as hell and last forever (sprag failures notwithstanding lol) but you pay for it with all the power they absorb due to the heavy internals.