i'd make sure that there is not something in the BB shell on the frame that is preventing the cup from going in. IOW, it may not be the threads. i'd check any sort of derailleur guide bolt that may still be on the frame's BB shell, if there ever was one.

sometimes just the threads on the frame's BB shell for use with a BB derailleur guide, if applicable in your case, can protrude into the BB shell's threads, if they were cut after the shell's threads, and cause a problem too.