Basically, better internal parts...sort of like the internal differences between a 727 behind a 2bbl 383 and one behind a 426 Hemi. :) Extra clutch packs, stronger planetaries, different governers, etc. I recall the lo-po 200's wouldn't lock the converter at WOT.
If junkyard picking, you most likely donor for a good one would be a Monte Carlo SS (look for a third brake light...that's 1986-88) or a Cadillac Fleetwood with an Olds 307 (third brake light is 1986-90, composite headlights are 1987-90). If the engine has the oil filler in the valve cover or has EFI, it's a Chevy 305, NOT an Olds 307...the Olds will use a separate fill tube in the front of the intake and all used Q-jet carbs.