Time to Fix It Right

Ignition timing at idle?

Sounds like it may be low, big vacuum leak or carb funneling fuel in unmetered.

Good timing specs takes care of LOTS of carb issues. No reason a magnum 280H cam can't idle at 750-800. Something else is causing the issue. It's bouncing all over because the mechanical advance is likely active at 1500 rpm.

It's going to like 16-20* initial timing is my guess