A couple more ideas:
1. I dont think thats too big of a drop. Typically that is what I see, about 900rpm neutral, 600rpm drive w stock converter. But, it should still be happy.
2. Is your vac advance ported to intake or spark port? In other words, I like spark port where you have NO vac advance at idle. Otherwise, when you drop it in gear, vacuum drops so you will lose advance, causing even more RPM drop.
3. To check for vac leaks, especially baseplate:
* Back the fast idle screw WAY out (count turns) so you can completely close choke plate without bumping the fast idle screw.
* Start & run, choke plate open, warm engine. Now slowly close choke plate. Engine should keep same speed, then slowly start to die out as the plate gets to be almost closed.
* If it speeds up when the plate is about 1/8" from shut, that is classic sign of a vacuum leak.
* Try both mix screws 1-1/2 turns out. That will likely be TOO rich (normal is 1 turn out or less) but just try it.
