Big Block C20 Running Rich...Holley 750

[SavageC20]

I have a Holley 750 ( https://www.holley.com/products/fuel...arts/0-80459SA ) on the 454 in my c20 and it has run rich since I put it on there. To the point where it burns the eyes when the truck is sitting there idling.
I have set the idle mixture screws with a vacuum gauge to achieve the highest reading. I am thinking that I might need to step down a few sizes on the main jets. I know I can take it to a shop and have it tuned but I would rather learn something and save some money.

The carb currently has size 72 main jets. It runs fine at partial throttle and wide open but I know it's too rich. Plugs are black...

Any suggestions on where I should go from here? I don't know much about carburetors on vehicles but I have rebuilt and messed with several on dirt bikes, quads etc so I understand the concepts.

Engine Specs:
-454 bored .40 over
-Stock Crank and Rods
-Flat Top Pistons
-049 Oval Port Heads (Stock Recon.)
-Edelbrock Performer 2.0 Intake
-Stock Cast Iron Exhaust Manifolds
-2.5" Dual Exhaust
-HEI: Timing is set at 14 initial and 35 Total w/ vacuum advance unhooked
-1.72 Roller Rockers
-Hydraulic Flat Tappet Cam Specs:
-Dur. @ .050: 204 214
-Adv. Dur.: 282 292
-Lift: .476 .501
-Lobe Separation: 108 116