1972 402 stumbles under load

[1972_C20]

Ok, I'm about to light this thing on fire. Truck idles ok and cruises ok. Once the 4 barrels even start to open up, it falls on its face and backfires out of the exhaust and the carb. It's different every time. Things I've done.

Cam ran flat, so the cam is new. Yes it's broken in correctly. Truck ran good after new cam was put in. Started acting like it was starving for fuel, so I installed an electric fuel pump. This fixed the truck for a week and it started sputtering under load again. Rebuilt carburetor and did not fix the issue.
Ended up replacing the carb due to it leaking fuel from the throttle linkage. I replaced it with a 750cfm summit brand carb.
Timed at 16 btdc
Adjusted valves with engine running.
New ignition coil
New spark plugs
Spark plug wires, cap, rotor, and ICM are no more than 6 months old. Cap is not cracked. Dielectric grease on all wires, plugs, and connectors.
Fuel filters are brand new.
Fuel pressure is 5.5
Vacuum advance is working
Mechanical advance has been cleaned of all rust and moves freely.
Replaced power supply wire going to my hei dizzy.