Given what you want to haul I'd suggest the flat bed and maybe duals rather than a box.
There aren't a lot of options that are likely to make a big difference for your application. If fixing the truck up includes making modifications I'd make more suggestions, but just stock, limited to a Chevy, there just aren't that many options that will make a whole lot of difference.
I will say that you might look for one that already has a 261 or a GMC 270 or 302. You're likely to want more power then a 216 or 235 are going to offer. Those bigger motors won't be in a Chevy unless someone modified it. That's what I got...! And if we ever do get it running, we're gonna swap in a 261.
'76 F-350 Super Camper Special (140" WB), Ranger-XLT, Fast 390 V8, 4-Speed, Gun Rack, Brush Guard, Dual Exhaust, 3.73 Gears, 10K GVW.
'74 F-250 Super-Cab (155" WB), Ranger, Rebuilt 390 V8, C6 Auto, Tachometer, Headers, '77 Grille, Kill Switch, Single Exhaust, 4.11 Gears.
'59 Chevy 38 Apache Stakebed (Not running), 235-Inline-6, 4-speed, Gun Rack, 3 Tanks, Dual Rear Wheel, 9.5K GVW & no fancy stuff.
'99 Mitsubishi Fuso 5-Ton Delivery Truck, Diesel, 4-Speed Auto, 1-Ton Lift Gate, 18FT Box.
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