another M-1 vs Airgap question

Thats where you need to be honest with yourself. If you go to the track once a month for a test and tune night to see how fast the car is and the rest of the time you drive on the road then a smallish carb is what you want.

A vacuum secondary carb is more tolerant of being oversize but it still better to use 80-90% of the capcity of a 650-700 cfm carb than it is to use 70-80% of the capicity of a 750+ cfm carb.