440 six pack high idle

This Dieseling points to a vacuum leak, or a carb hanging open and letting in air. Like an outboard. I really think this is your problem.
Engine off, disconnect choke and have center carb at idle. Remove one outboard linkage. Manually open and close that outboard carb. Make sure it opens and closed without sticking. Coles that carb. Now that linkage should slip in the lever on that carb, if not turn it to adjust length. Do the other one the same way.

when it works right you don’t need that solenoid.

yes Weak dist advance springs Can do this. Do you have a stock dist to put in for a test. What coil is on it? Swap on another for a test.

The next problem is you could have a cracked intake. The aluminum ones will do that. Seen many for sale that are welded up in the heat crossover. Not common, but possible on aluminum intakes.