Non-vacume ported spacers for carbs in hight?, type?, material, and gains?

Years ago I used a LD 340 intake on my mild 340. 13.8 quarter mile at just over 100 mph. Tried a 4 hole and an open 2 inch spacer. Picked up 1 tenth with each 1. No difference between them. Seemed to run exactually the same. Had a 750 competition Carter carb on top. Kim. 67 dart GT 3300 pounds. 3.55 gears