The first things I think of when I see what you are looking at are, that engine should be worth $1000.00, and if the rest of the truck is worth nothing is it worth having. Granted I have not seen the truck, so you may have found someone who just doesn't know how to price things.

Second, you are trading a twin i beam truck for a solid front end. There are gains here in the durability, but losses in the ride.

I would proceed cautiously, and look carefully at the older truck because the steps are known to rust much worse than the 65-66 floorboards.