high speed miss

CHANGE THE OIL NOW! How old is the fuel pump. If it's busted the diaphram, it may be leaking fuel to the crankcase.

Get the baseline carb adjustments set. Float level, 1.5 turns out on mixture screws.

Disconnect and plug everything vacuum at the carb/manifold. You don't need the vacuum advance at this point. Plus, this gets rid of potential vacuum leaks.

What's your initial timing?

Could be any number of things. Start with getting a good baseline timing set up. Say 10-12 minimum before TDC. If you have a vacuum gauge set it to the highest point advancing the distributor then backing it off one inch, while keeping the idle RPM constant.

Do you know how much mechanical advance is in the distributor.

The internal distributor plates are known to move around a bit. If they aren't solid and the air gap is changing, it could cause a miss. Bad cap/rotor, rotor phasing.

Since you say you have to back out of the throttle, what exactly happens. Can you roll back into the gas and it picks up. Does the thing basically shut off at 3K?