Are the carbs vacuum secondaries or mechanical? You are correct, that tunnel is a problem on the low end but as long as it looks good, who cares?!
I have always had great success in timing specialty engines to which the factory manual does not apply by using manifold vacuum. Disconnect the vacuum advance and attach a vacuum gauge to the hose. Then start twisting the distributor and adjusting idle screws until you nave the engine idling at desired rpm and maximum manifold vacuum. This should be pretty much optimal. Reconnect the vacuum pot. Test drive and if then engine pings at any point in it's rpm/load, back out a degree in the advance and try again. Back out only as much as necessary to stop the ping.
Also, it is critical on Holleys on all engines but especially so on a tunnel ram to adjust the squirter cam just tight enough so no matter how little it moves it squirts. If it is adjusted loose, you will always have a bog as the throttle opens but the slack in the squirter arm delays the extra gas. Another possible cause of a bog is mechanical secondaries that open the carb way too much too early. Vacuum secondaries are better for the street virtually all the time 'cause they can be tuned to open big only when needed. I know guys will pipe in with anecdotal stories of 1000cfm double pumpers getting 35mpg and perfect throttle response but in general, on the street with the wide variety of load conditions, vac secondaries will win nearly every time. This is a problem w/ tunnel rams doe to the big plenum which isolates the carbs from the cylinder pressure signals that are used to signal the carb what to do.