I'm looking into buying a used 454 but i dont know if i should keep my old 2 speed powerglide or get a used th400 for $200 from a friend???

i think you would probably rip that powerglide apart witht he 454

I really doubt he'd rip it apart! The only thing that would break in a stock 'Glide is the high gear hub if the motor makes some power and the tires are dead hooked on the 1-2 shift. You can run a stock 'Glide with nothing other than a good input shaft and hub, and valve body stuff, behind 800 HP with decent reliability. For a heavier street car, you'd be happier with a 3-speed though. 400's only need a good intermediate sprag, and shift kit stuff, and it will live forever too.