Couldn't find anything on this here at FTE so here goes.
I did a compression test on my 79 and the numbers are... concerning, to say the least. The results are as follows by cylinder, roughly:
1. 65
2. 65
3. 60
4. 65
5. 65
6. 70
7. 65
8. 70
I figured it was low, but the numbers were all pretty close so didn't think it could be that bad, maybe 20 low but hey even, right? After looking up what the m block is supposed to produce I thought my gauge MUST be shot because the specs are supposed to be 100s-140. Tested my gauge and it checks out. The engine runs okay and the numbers ARE close so is there something more likely than my cylinders are THAT sloppy? Bad timing chain or something? Any ideas?

 

Test run with throttle blocked wide open? And all spark plugs out?

A squirt of oil into a cylinder should show a big boost in pressure for the first few revs if its rings/cylinder wall.