Be aware that the label, "rebuilt" or "remanufactured" means different thing to different shops.
If you see a shortblock for <$500.00, does it have used pistons in a honed bore? If not, I don't see how the shop could afford to bore and install fresh pistons/rings, turn the crank, install bearings, etc. and still turn a profit.
You can take a chance on a junkyard engine that's in running condition, hit up craigslist, shop the machine shops in your area for engines that weren't picked up (and price the rebuilding of your old engine while you are there), shop the newspaper classifieds, look for a running vehicle w/a 350 in it for cheap- the options are many and varied.
BTW- A '72 350 truck will have at least a 2-bbl. on it.