Engine swap advice

This is where having a good timing light and a vacuum gauge can help. If the timing is jumping around at idle, it could be as simple as a bad distributor or loose point plate or more seriously, a worn out timing chain and gears. The vacuum gauge can give you an indication of a vacuum leak or poor air/fuel mixture as well as a burnt valve. Before you do ANYTHING, give it a valve adjustment and compression test to make sure the basic engine is sound. Otherwise you're just wasting your time and money guessing.
Thanks, Mark