Got a problem with a 1987 Ford F150, six cylinder, standard shift that won't fire. Had previous problem and mechanic told me he had to reset a relay. Dumb me, wasn't watching close enough. Popping the clutch won't help. Can you??

Truck is 1991 Ford F-150. 351M V-8 engine, Auto trans, 4 wheel drive. Problem: 2 cylinders have [U]very[U] low compression. Facing the engine from the radiator they are the two front on the right. These two cylinders have around 5-10 pounds, the rest are much higher (don't remember exactly what they are) and are very close to being identical. I think there may be burned valves, but not sure how to find out. I have heard that if I squirt oil in the cylinders with bad compression, and the compression improves, then the rings are bad. If the compression stays the same then there are burned valves. Is this true? Is there any other way to find out, short of taking the heads off? I have a 1978 Ford 150 with a 351M engine. Will this engine work in the 1991 body?