We're resurrecting the Pumpkin into the Perrywinkle beast. Crash destroyed the front end at the June Cove trip. We're using the suspension and dual cases from the Pumpkin to make his truck solid axle. And the Pumpkin diffs are going in my 80 to regear and lock it :)
We put a new Toyota short block, and timing set in. ARP head studs and a Cometic MLS head gasket. Otherwise stock motor. We're still working on the overall build so it hasn't gone back to daily driver status yet.
The only con to this is that decent water pumps are becoming hard to find. Most of the ones available now are garbage. It'd suck to replace a decently healthy Japanese water pump with a crappy Chinese one.
With backfiring and sputtering, you may be running lean. Running lean means running hot. Also check the timing to make sure you aren't overly advanced. That could make you run hot as well and would be worse moving faster under load.