I just remembered a local chromed C-85 engine that would "sag" on
takeoff after an overhaul. It was supposedly traced to inadequate ring
gap. I find it hard to believe in your case though after your amount of
time since OH.

You might check the carb float bowl fuel level with the clear flex
standpipe connected into the carb drain port method.

Make sure the correct rocker arms are in the correct locations. On
some Lycomings they are different for the intake & exhaust. The
exhausts have oil squirt holes in them as I recall.

I'm getting outta ideas.....