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.
