I would think that the progressive linkage system with the two center carbs as primaries would function reasonably well; as stated previously, with lower throttle angles and hence lower manifold pressures, and it seems that the Edelbrock F68 does seem to exhibit adequate area under the carburetors for communication so there should be adequate air motion and not be a problem.
As far synchronizing the carburetors, each has his own preferred procedure, mine begins with the initial set-up for start-up. Start by dropping all of the linkage from the carbs, then close down the throttle blades until each seats in the bore, now raise each throttle an equal sum with each throttle stop screw. One can choose to utilize feeler gauge stock or better is rod wire to establish the butterfly blade to bore or perhaps just utilize the screw pitch on the throttle stop screw, to present 'some' by-pass value. Remember that this sum that your adjusting for is idle control, which the correct some will be established after start-up, but also by realizing that idle and the low-speed tip-in of the multiple barrels is the most critical transition area for the atmosphere of fuel & air this for smooth transition of engine response and acceleration at low speed.
Then reattach the linkage starting at the primaries or the carburetor(s) closest to wherever the throttle pull emanates and attach each linkage section so as it does not affect the at rest carb throttle positions; and if one-to-one, or for those units being of secondary intention, each time adjusting to remove any freedom on travel in the pull direction of each instance; slack in the throttle closed position won't matter as in each instance the throttles will set on the stops, but with it tight in the pull direction the intent is to have all throttles opening together (synchronized).
Now your ready to start the engine, beware of excessive idle speed as there are multiple throttle bores in play perhaps presenting excessive open area, so begin dropping the throttles by the use of the throttle stop screws equally (if progressive that of the secondary units first) until a reasonable idle speed is had (yes, you'll have to revisit the primary carbs also). Double check that the linkage is not influencing one or more carb butterflies to be positioned other than your intent; this is often established by just dropping the linkage again and rechecking the previous effort of its' fitment.
And now, pull-out your sync. tool and test for how you did for this initial start-up setting-up!
There will be variations on the endeavor as for example: if progressive linkage is utilized you might want to drop the linkage again or just establish that it doesn't act upon the secondary units, in order to adjust and synchronize the two primary units together. You'll want to have the secondary units shall we say 'on-line' some as one wants fuel to be in motion in the carburetors at all times as this reduces the tendency for delay of function particularly at tip-in, and this participation will effect particularly idle control so the trick, and particularly as the number of carbs increase, is to establish the throttles' most closed position against their individual throttle stops where it's is still at work, but minimally, this giving the greatest responsiblity to the primary units as intended.
As far as establishing that the carbs are all 'exactly' at W.O.T. when such is demanded though appreciated is actually less of a concern; this is simply because the effect on the engines' function at high speed of a few degrees of throttle angle is significantly less than the headaches one will experience with such at low speed and low or closed throttle angles. But do establish that this value is reasonable, particularly as if not it would seem one might have throttle linkage issues which may also be presenting other problems.
And now pull-out your sync. tool again and begin the fine settings for synchronization; if one to one linkage then all should be equal; if delayed linkage then the two primaries should match and be of a greater flow sum than the otherwise matching secondary units.
And..........at rest, all carburetor throttles should be setting firmly on each of their own throttle stops.
Scott.
