[Jac Mac]

When you look at a 4bbl Holley closely the sight plug for fuel level in the front (primary) bowl is slightly higher than the rear-- with the carbs mounted @ 180° from the norm plus the forward downslope of your intake manifold I would imagine that the fuel levels in your application need a rethink from what Holley recommend.

I seem to remember that the floats need to be sealed with a 2 pot epoxy type glue or paint, but in this world with changing brews of fuel etc that may no longer be correct.
The lack of whistles is likely to be your stall while braking problem.

Just so Im clear on this with your comment about cracking the second carb, are you running them staged ;IE one carb starts to open prior to the other or were you refering to the secondaries of each carb.
Also, Holleys idle on all four barrels of each carb, its just that only the primary barrels have adjustable mixtures on most models unless fitted at all 4 corners. You may be able to get a sweeter idle by closing the secondary butterflys idle setting slightly, or in some cases by opening them slightly if the cam is fairly radical.----screw in base of carbs ( on manifold side ) below secondary diaphragm link.