I'll assume your ignition timing is locked in, no mechanical or vacuum advance and your carb is 4-corner idle adjustable.

Idle is too lean.
Make sure you have balanced the idle speed stops front and rear (indexed the throttle blades).
I'd start with the mixture screws 1 1/2 turns off seat and try it.
Back all mixture screws out 1/4 turn at a time, hopefully making adjustments to the front and rear idle stops evenly as you go.

If the mixture screws don't respond either way, you're already pulling fuel through the transfer slots, increase idle air bleed size while closing the throttle blades evenly.

I allow a maximum 300 RPM drop when the trans engages, and it must remain clean and crisp after being free revved or coming off loaded power.

^^^^^This.

Vacuum leak will cause the same - carb base plate loose from main body, carb flange loose, intake bolts loose, etc.

So...update. Engine builder said idle mixture should be out 2.5 turns. Hes also sending out another carb to try. Not saying no vacuum leak, but builder said they ran it on dyno before putting back in car.

For the guys thinking its a vacuum leak., what should a blower motor be pulling for vacuum at idle...in park.

I think you’d be better off smoking the intake and seeing if/where it comes out.