Mick, have you tried adjusting the mixture screws in the baseplates of the end carbs? Backing them out maybe 1/8 turn each will make a big difference and take some of the load off your center carb. I have set up more than a few 6 paks and even with big cams, I've never had to open idle passages in the center carb. Good luck!
Best idle on a small block Sixpak setup with an XE268 cam ended up with the mixture screws on all 3 carbs out 1 5/8 turns. Covering the corresponding air bleed one at a time will tell you which direction to go with the mixture screw. Never had to drill anything.
I have done that following the writeup from Dayclona, and have the end carbs, as far as I can tell, adjusted properly, in that when I cover the air bleeds one at at time, there is no change in the idle. There was when I first put the carbs on, so I know there was a change.
