Everything here sounds to me like a carb issue, not spark. Especially cranking down the mixture screws not having much effect. Do you have a vacuum guage? Thats the best way of tuning a carb short of putting an O2 sensor in the exhaust. Without one your best best is to tune mixture screw for highest idle, reset idle screw, recheck timing, then reset mixture screw again and recheck everything still it stays the same. Also make sure the choke is fully opening.
Pull the plugs, sooty black-running rich
Check for vacuum leaks, brake booster diaphram and distributor vacuum line are often overlooked culprits.
Fuel boiling in the lines or carb bowls can cause theses symptoms as well. Does it have a fuel return? Lines routed too close to exhaust.
I don't know anything about the Holley quickshot, but I played with lots of older Holleys. Pain in the arse, everyone of them. Way too much adjustability and super finicky. Floats bounced on every big bump offroad causing a floodout, Steep climbs caused bowls to overflow, floodout. Had to rest mixture and choke settings at every season change. Tried all sorts of tricks, spring loaded needle and seats, overflow tubes, etc. Ended up going to an Edelbrock Truck Avenger carb and having WAY less issues.
What did you put in for a cam? Anything radical? Still idles nice or lopes?