How much RPM drop from park to drive with stock type 340 convertor

340doc said
Lots of good info, and some things for me to check, thanks to all! I'll answer some questions and provide more info. Stock 1970 AVS carb with 2 idle mixture screws and the idle screw on the shaft. Timing is at 14 in park, I'll ck it in gear as mentioned. Transfer slot is currently set at square. I'll open it up a bit and back timing down to set idle rpm and see what happens as mentioned. Mixture screws are open quite a bit, its what it wanted. I actually feel the stock jetting is lean, If I had a old strip kit I'd probably fatten it up a bit. Float level has been checked. Vacuum is 14 in park, Ill ck in gear. Idle speed in park is 975, drops to about 650 in D. I'll do some work and report back.

Sounds like a good plan!
If you can do it safely, check timing at 650 rpm. IMO 14 at 650 rpm would be close, and Rob suggested 15* would be good.
You may find a huge diffence in timing from 975 to 650. Only way to know will be to measure.

And don't worry about the jetting/rod for now. As long as the rod/jet combo is significantly larger than the idle restrictions, it will have no effect on the idle and off-idle.
Carter idle system is a little different than the typical Holley in that it uses a second air bleed and restrictor. So general rules of thumb for a Holley about 'square' or 20-40 thous. slot exposure don't apply. However the concept is still valid: More t-slot will provide richer idle. Too much or too little will cause flat spot or delay.