Colt's have two different sized takedown pins. They can be mixed & matched with a variety of adapter pins & bushings. Available from several vendors. Not a problem once you get the adaptors.
The ATF sez they want 1100F or higher melting point for anything that keeps the threads covered. JB Weld will do many things, but that's not one of them. "better than baling wire" - A Kansas farmer.
The illegal parts of the upper would be the barrel threads, a flash supressor and a bayonet lug. Zap those & you should be OK.