I think I'd stick with the early chassis, and add a leaf or two in the front springs, if necessary. Doing a body swap is going to involve a lot of work, and there's no guarantee it would look right when it's finished. Cutting and splicing a frame is a tricky job, too. We do it on the roundtrack cars, but that's an entirely different situation than building a street machine. BTW, the weight difference between the stovebolt and the 350 isn't much. Those old sixes are HEAVY!
Jerry