I'm with Rod. I really doubt a missed re-torque is causing your problem. It might account for some oil leaking down the side of the block a year from now, but not a compression leak.
I would be far more worried about the valve imprint in your piston. Assuming the pistons were replaced at the last rebuild (in other words, you're not looking at prior damage), you HAVE to figure out what's causing the contact.
What's the bore size on this engine? 4.2 or 4.4"? Have the machine shop check valve recession while they are looking at it. I don't have an exact spec in front of me, but if memory serves, the valves should be around .050 to .060" recessed.
If that checks out, piston protrusion would be the next thing I am looking into. This will also determine which head gasket you use. More on that next week after I do some research.