I have a Professional Ordnance type 97 carbine. I have had two "material" failures that have been fixed sucessfully. I suggest using some sort of metal "bridging" material, like steel wire in addition to using an adhesive like JB WELD. If it were me I would drill a hole, or holes, in each side of the break. Then glue in the steel reinforcing material, and glue the joint. My lower split along a mold mark down the top center of the ring into which the buffer is threaded. I made such a repair, and it has survived several thousand rounds thus far. Good Luck.
As an additional thought, not knowing exactly what upper you have. Some uppers and lowers do not have perfectly "standard" hole spacing. The Pro Ordnance uppers, for instance, will not mate with any other lower, as the hole spacing is different. Once you make the repair, check carefully to see that the spacing on the upper matches that of the lower. It may be that the fracture of your upper came about because the hole in the lower was offset slightly, and the pressure from the pin, if it was steel, may have been too much for the upper to bear.