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Easiest place to find the 4b or 4bt is in old bread trucks, Dolly Madison, Frito Lay, International Brands corp. All of them run the P30 Chevy chassis with the 4BT conversion.
http://www.dovebid.com often has trucks for sale with 4bt or 4b. search for cummins, 4b, or 4bt. Sometimes they have them sometimes they don't. You can also search sold items.
I like the idea, jade79. You might want to consider making some mods to the truck, however. Like fiberglass body panels, lowering kit, chopped top, drill a bunch of holes in the frame, lightened driveshaft, lightweight cover on the bed (you won't be able to haul much, anyway), racing style hard plastic bucket seats, and replace steel with aluminum wherever possible. If you decide to go this route, be sure to post pics!
speaking of Cummins and gas mileage my cousin's boss has a 2000 Dodge 2500. 2 weeks ago when they were hauling round bales they got 22 mpg......that's with a 30' flatbed goosebeck loaded down with 15 6x6 round bales. the truck has a Bully Dog chip, 6 speed short throw, 4x4, 6 inch lift, twin turbos, upgraded injectors and pistons, 5 inch straight pipe with 6 inch tip, Banks power package, so much other stuff I can't even remember.