Do you think I could use an old timing chain and gearset from a junk yard car?  

not if you want any length of service out of your engine .  pop for a new timing set its not that much really  

Well, right now my options aren't cheap either way for a roller hydraulic cam.  An LT1 or Gen I cam both cost the same, around $260 new.

If I get an early double-roller timing set for $32, a thrust button for $10, and a timing cover with a thrust plate for $126.  I'd also have to grind the block a bit to make it fit.  That puts me at $428 with an altered block.

If I get a late timing gear set, non-roller, it'd cost about $193.  That'll put me at $453 with an unaltered block.

Right now, I'm considering:

- buying an early double-roller and machining it to fit my engine
- finding a used LT1 timing gear set and replacing the chain  

Ahaaaa.  Nevermind.  I just found a stock replacement for $56.  

Used parts on a new or rebuilt engine not a good idea at all.