I don't think any amount of metering screw tuning is going to solve a 400-500 rpm drop from park to in gear. Something is VERY wrong.

Isolate the carb from everything else. If you have anything vacuum related hooked up to the carb, pcv/PB, remove it and block at the carb.

This is the same basic build and it idles at 850rpm in park and 750-800 in gear. It would kill the tires with a 2500 converter and 2.94 gears.