The lifters are already full of oil and under pressure, so when I tighten them down I am afraid of all that pressure tightening them down might strip the threads in the aluminum heads.

Any thoughts on this?

FYI: all I did was take the heads off to change the gaskets, didn't drain the oil, nothing was done to the engine, etc. Just replaced gaskets and sealed what I could off and gave it a quick paint job.