I'll tell you what would make an excellent as well as eye-catching motor in a truck is a tuned port 350 out of a late-80's Corvette or F-body. Especially if it has the excellent 700R4 attached to it.
This is an extremely torquey motor that will pull up to 400 lb-ft at about 3000 rpm if you cam it right and improve the compression with steel shim head gaskets. And with the 700R4, you can cruise at just above idle. I get 22 mpg with mine in the IROC, cruising on the highway.
They clean up nice with either paint on the runners and upper plenum, or a good polishing job.
I've seen the combo at swap meets for as low as $800, which is really a bargain.
