IMHO it's a bad "investent" to build any post '73 pickup. There's just too many of them still out there. As for the long vs short bed, a similarly built short bed will bring more $ 99% of the time (of course 75% of all statistics are made up
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If you're building a vehicle to sell for profit, just look at the auctions/trader papers/ebay/etc...most often the big $ is in pre-72 Chevelles, Camaros, Corvettes, tri-5's, etc. Trucks typically don't bring in the major dough, although they do cost as much to restore as those cars.