I have a 10.6:1 compression 360 with the same Comp XE268H cam. The engine wants about 17 degress BTDC for intial timing. I have the mechancal advance set for 16 degrees for a total mechanical of 33 (magnum heads like less total than open chamber heads). The vacuum advance is adjusted so its all in at 14"Hg of vacuum.

You have to get the timing right before you can even think about messing with the carb.

FWIW, I am running a Holley 670 Street Avenger on my engine and it was just about perfect out of the box. I tried larger carbs and the low rpm throttle response was not what i wanted for a car that is driven approx 7500 miles a year on the street. The 670 gives up nothing to the 750 I had on it on the top end either.