Well, it's time to put a bow on this thread.
As you recall the newly rebuilt , original style, glass bowl fuel pump works perfectly. This was after two brand new ones failed - one in only a few hundred miles, the other didn't work straight from the box.
Stumbling bog and backfire persisted and after ruling out ignition and timing, the culprit seemed to be the freshly rebuilt carburetor.
As last ditch effort before removing the carburetor and looking for internal problem I sprayed every orifice with carb cleaner and put some puffs of compressed air into the the fuel inlet.
Problem has disappeared, Truck is back to running its reasonably smooth and quiet self with no bog on acceleration or backfiring, etc. Hopefully this is the end of it.
The entire fuel system is brand new so if there was debris finding its way into the carburetor I don't know where it could have come from.
