Use a cylinder stop in the #1 cylinder. With the battery disconnected, rotate the engine one direction until you hit the stop. Mark the balancer at the TDC mark on the timing tab. Rotate the engine the other direction until you hit the stop again. Mark the balancer at the TDC mark on the timing tab a second time. True TDC will be halfway between your two marks. If the balancer has not slipped, this will be at the TDC mark on the balancer. If it has slipped, you can use the TDC you found to time the engine but once a balancer slips it usually continues to do so. 340's were notorious for balancer problems and I timed more than one 340 using this exact procedure.
This works great... if you did the same procedure to verify TDC when the engine was fresh. Or at any point before your concern, so that you have a baseline to compare to. I've checked brand new engines with all new parts and the timing mark on balancer doesn't line up with the timing tab on the front cover.
