If I had to guess, I'd start with the ignition, if you find the idle
mixture is about right - with the higher compression the ignition is more
critical - put the stock Dist back, new plugs and new wires (and dist cap,
etc) and see if it's better.

I'll second the motion. My old man had a saying for old cars, "Its always
the ignition, when you're sure its something else, its in the ignition".
Plus you can see the other shop has beat carburation and valve timing to
death with no effect.

I'd tell this guy, strickly time and materials, no promises. Or, he'll just
be mad at you, also.

Karl