A rebuilt engine is just that! "rebuilt" the standards for production rebuilding are the same from Reccon "autozone&advanced" as Marshal "Nappa". A rebuilt engine from a parts store is a gamble because the builders are not as through as a mechanic would be. major engine rebuilders dont care, basicly if your engine blows up they will send you another at almost no cost to them.
Yea you can get a rebuilt from nappa or azone but thier bascily the same. If you want a reliable engine keep that 900$ and save 600 more and get a gmcrate engine.