Way to fat on the part throttle cruise, maybe others issues also. Will it start and idle well when cold? Also will it sit and idle for 2 or more minutes when warmed up in gear and not die? What jets are in the center carb. and have you tried leaning out the mixture screws on the out board carbs? What carb. list numbers are yours?
Last thing is those plugs are way to hot for a stock compression pump gas Magnum 440, is your compressio ratio stock or lower?
You can make six paks run really well, with some effort