I'm in the process of putting together dual quads for a 318 - am using the paired progressive 500 AVSIIs from Eddy. They came with an instruction manual that shows the differences between the primary and secondary carbs. (avaliable on line) No reason you can't modify the one you have to Eddy's specs and then buy the other one to match. Mine will be on a Edelbrock D-66. I have had a few progressive DQ users suggest that you use the rear cab as primary. That puts the primary two barrels on the primary carb closer to the center of the system and leads to better distribution at low throttle between the front and back cylinders, rather than feeding the back 4 cylinders by drawing fuel all the way from the front of the manifold.
PS: I'm running two Eddy 500 AFBs on my 340 with an STR-12. You can't run the carbs progressive on the crossram so they are set up opening simultaneously and they run great. Since opening is controlled by demand on AFBs an AVSs, I think that that helps with potential overcarburation. If that is true, you might get by with putting your set up on the 273 to see what it does.
