Idle RPM Problem
The following things can cause an engine to drop excessive RPM from neutral to drive.
1) Not enough initial timing for the combination. Very common
2) The butterflys being out of adjustment.
3) Torque converter too tight for application.
With a cam as large as you're running you'll probably need at least 16 degrees initial timing.
Trying to idle about 900 RPM, remover the carburetor, and set it so you see approximately .020" of the primary transfer slot, (looks like a little square when set right) then set the secondaries so you just start to see the slot.
This should be a good baseline to start with.