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

I'd set timing at 15* and 750 rpm and see what you get. It should run fine right there with minimal drop if everything else is in order. As Mattax mentioned, check timing both in gear and in park, they should not vary.

Need to make sure your total number doesn't go too high if adding initial timing. If it does, you need to limit mechanical advance.