If the 250 is in the truck and loose it could be a pretty good runner. I just removed all the smog stuff on the 292 in the 65 I bought from Don King last Stovebolt reunion. I removed the Octopus of little lines that go in the top of the head and plugged them all with Brass plugs, Plugged up the rest of the smog stuff on the engine and rebuilt the carb and it's a strong runner.
Like other's said if the S truck V6 is a 4.3 it's basically the same as installing a small block just minus 2 cylinders. If the V6 is the smaller one's he'd be better off keeping the 250. And the stock 4 speed in the truck should bolt right up the a 4.3.
Mongo
