Drops 500 RPM when I put it in gear

Worked on it last night. Set total timing at 34. This made the initial 16 (MSD dist). It reaches the total timing around 2700 to 2800 RPM. Then I adjusted the front and rear carbs by maxing out the vacuum gauge. The max reading I was able to obtain on the vac gauge was 15.5 (little low?).

I have yet to drive it but hope to do so this Sunday.