To me the plugs just indicate you are not burning
oil. You could be cruising lean, accelerating lean, lean at idle.... you don't really know. I'd guess from your carb card you are pig rich anywhere the lean spot is not between your idle and main circuit, hence the clean porcelain and tip and carboned threads. Without the a/f gauge your guess is good as mine.
tried to edit my above post--if it wasn't evident, the secondary PV on the OEM Holley that came off the 400M was plugged. I used 74 main jets all around on the HP 750. You can do either. I'd also prefer the single plenum intake over the dual.
Also just an fyi, the Holley Boss 302 carb main jets were usually down around 70, and maybe they used a 780 cfm.