Having toured GM's remanufacturing facility for the 4.3L (SRC in Springfield, MO), I personally would be more than hesitant to buy a reman engine from someone other than the OEM. They had pallets of crankshafts sitting there that didn't meet GM's spec to be rebuilt, but instead of scrapping them, they would sell them to other reman houses for use in their engines. I'm not saying that it won't last 100k miles just because it doesn't meet GM spec, but it makes you think about what else they're getting away with.
If the Jasper is only a couple hundred cheaper than the FoMoCo rebuild, then go with the Ford Reman. That's one you can be sure will last another 150k miles.