904 vs 727?

Really? What's different in a 904 vs a 727 where the lighter duty trans wouldn't explode? From what i remember they're pretty similar inside except everything is just bigger within a 727.
And money wise I'm gonna need a converter either way, and I'd definitely have to go thru the 904 since it was acquired as a "good core" with history and whether it works or not is unknown.
Only thing within the 727 that's got me wondering is knowing that it is (was) a L/U version and that part isn't working though it works great otherwise.
And why that is the case, what it would take to get it working, whether it would be worth messing with to get that part of it working.
I did change the fluid in the 727 when I 1st got the truck running, had to because the pan gasket was leaking like a sieve. The old fluid was clean and red, no crap laying in the pan.
something bout the sprag spinning astronomical rpms and coming apart,..do a search in the transmission section,..happens when drag racing not in regular duty use....like i said,..not sumptin you need to worry about....only mentioned it as why folks might prefer the 904....