I would suggest at least dianosing the problem on your current engine.

In the least, I would suggest going through the hassle to drop the front axle and the pan to see if the thrust washers are there. If they are, you should be able to replace(with new ones) the thrust washers with the block still in the rig. Not easy, but do able by pulling the #3 bearing cap.

This, will at the least prevent you from destroying the engine. If you continue to run it like it is, and if it is indeed the thrust washers, then you will ruin the entire bottom end. Poof, gone.

If you do get the washers replaced, it will buy you some time while you build something else. And, it will probably ensure you have a rebuildable core to work off of.

If it were me, this is something I would seriously consider doing if I had to keep it mobile while another engine was being built.

Good Luck,
Michael