Paul makes a good point. 'Rebuilt' can mean a lot of different things. Try to obtain any documentation available so that you can gauge the completeness and value of the work done.
The value of a 'rebuilt' motor depends upon what was done, how it was done, what parts were replaced, the quality of the replacement parts, and who did the work.
Some examples:
1) WHAT WAS DONE: overbored cylinders? straightened, rebushed, resized, rods? hardened valve seats?
2) HOW WAS IT DONE: if the cylinders were bored oversize, was a deck mounted boring bar used or were the cylinders bored with the boring bar positioned off the crankshaft mains? if the rods were resized, did the shop have the equipment capable of maintaining center-to-center distances?
3) PARTS REPLACED: camshaft, manufacturer? pistons, manufacturer? valve lifters, manufacturer?
4) WHO DID THE WORK: machine shop work, engine assembler? -Their reputation and their knowledge of Ford y-block engines? Different species of engines have nuances that may not be known to someone going by a Motor's Manual, Chilton's Manual, or even a Ford manual
Hope this is of some help...
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York