As far as the differences between the one ton and the half ton frames,they used the same cab and front sheetmetal,so the main differences would probably be in the rail dimensions (thickness,height,depth) and possibly wheelbase.
I don't think it'd be a good idea to box the frame rails,unless you plan to really thrash the truck. The frame should be plenty strong as is,(unless there's serious deep rust in it),and boxing it is a lot of work for the marginal benefit you'd gain. If you decide that's what you need,it'll stiffen the frame considerably,so it'll probably ride a little harsher and handle a little better. Without the built in flex,I think it'll also be more prone to stress cracking.You'll also need to re-route any wiring,brake and fuel lines that currently run inside the channels,and make surre to leave an access hole for any bolts that run through the frame,or at least spot weld the nuts to the inside of the frame so you can remove and re-install the bolts once the frame is closed in.
Personally,though,my vote would have to be-no,don't box it.
Speed
