On a built 351C I used front 70's front & back 80's on a 650 Double Pumper Holley.
You have to play with it.
The fronts are easy, keep droppin em until it runs like **** when you accelerate then go up 2 sizes.
The backs, you just keep goin until it bogs when they open, or black **** comes out under WOT, then back off 2 sizes.
thats my rule of thumb anyway.
I';m sure there's more scientific methods, but it's worked super for me for 25 years.
Mach1steve
I didn't notice that you said a 750 VAC SEC....
thats different.
The secondaries use a metering block, and you have to buy the complete metering block I think to change the secondary jetting.
In would try going to a lighter spring in the secondary actuator, this will allow the secondaries to open a little sooner, which may work great for you.
Try a "Yellow" spring (they are color coded).