Further to Pipewelder's suggestions, I'd also recommend to check the threads after the weld is complete. Chances are (good chances, by the way) that the threads will suffer some warpage due to the heat input from welding, preheating etc.
Put the welded pieces in the lathe and pass the chasers carefully over the threads, using the proper lubrication.
Giovanni S. Crisi
Sao Paulo - Brazil