The term ‘rebuilt’ can be quite vague.  The mixed/incorrect engine parts would make me suspicious.  When poor maintenance eventually resulted in smoking/oil-burning, people often merely changed the rings, and then installed a set of used heads, rather than performing a complete disassembly & proper rebuild. I would want to verify the short block was indeed solid before proceeding with expensive upgrades (good bearings/balance, authentic rods/cam/etc.).  

6 VOLTS/POS. GRD. NW INDIANA

Good advice.

I would never spend a lot of money on a used or rebuilt engine, unless I either heard it run or knew the builder's reputation. It's not unheard of to get less when you pay less, but you don't always get more when you pay more.