1970 Volkswagen Van NEW engines in our very OLD Volkswagen

1970 Volkswagen van 1600cc mileage: 600. Hi and thanks in advance for your site. We have been enduring a nightmare with NEW engines in our very OLD Volkswagen. It began in May of "99" when my wife and I decided to put a new engine in our 1970 Volkswagen Van. I won't go into the awful details of how long it took, but in AUGUST! Our mechanic had the job finished. That engine lasted all of just under a thousand miles when it lost all compression in it. Mind you, we were very diligent about changing the oil and getting the valve adjustments done on time in all of these cases. It was determined that two of the heads were bad on the engine and they were replaced. Again, about a thousand miles after that, the engine totally blew out. This time they installed a new engine, which is now going out again after just 600 or so miles. My question, could there be circumstances within the vehicle that are causing these engines to blow? It's very odd, because the Volkswagen engine is so simple to begin with. The mechanic says it's not his fault, the engine company (GEX in California) says it's not their fault. We know it's not our fault, but we're stumped nonetheless. Thank you again, Tim and Debar.