"I'll have a go re-recording the separate L and R of a portion of the tape to test how much bother it is to keep in sync in a WAV editor for 20 mins or so. But not this week. "
I'll save you thr trouble. I've done this with a 2-hour music concert, though it was syncing audio to video (which was actually tougher for a variety of reasons, including losing sync every 5 to 10 minutes). Left-to-right would be a piece of cake for me.