I never seen a slant 6 that didn't run good. They're a dime a dozen. You should be able to find one good and running for 250 bucks or less. That should likely include the transmission too!
I don't ever recall seeing a slant 6 at the swap meet unless it was actually in a car for sale. Things like that people likely won't waste their time trying to put something that big in their truck that will get them so little money. You shouldn't have to go to the swap meet to get a motor, that should be an easy Craigslist item.
