the main reason i said an early 90's full size truck is because of the availability of parts. 350's, 302's and available at literally any junk yard, and they are very reliable motors. And they can go plenty quick, which shouldnt really be one of your concerns.

I can pull a 3000 lb boat 75 - 80 mph anywhere I want to go and still get 12 - 13 mpg. If I hook up the same boat to my 2001 s-10, I struggle to hold 60, and go from close to 30 mpg, down to about 14.