Show me a production-line reman engine, I'll show you a piece of crap. Every possible short-cut will be taken on the machining and probably on the parts selection.
Ask them if there's any warranty on engines installed "at home" by the vehicle owner. (i.e., not "professionally" installed.)
Ask them who pays for shipping a defective engine back to the rebuilder if you need warranty work.
Ask them how much time/rate per hour they allow to R & R a defective engine. Do they include fluids and filters?
Ask your shop how much time you're going to be charged to R & R that defective engine, and install the warranty replacement, including fluids and filters.
Ask them who pays for your rental vehicle while yours is tied-up waiting for the warranty-replacement engine.
What happens if it blows-up when you're a thousand miles from home?
If you have a minor warranty problem, how much do they pay the shop to take care of it? Is it anywhere close to the "shop rate" you're going to be charged?
The best thing about the GM "new" engines is not that they're "new". It's the warranty coverage: Coast to coast at any GM dealer; for up to 3 years (far as I know.)