I had a 66 cuda 273 commando that I overhauled back in the early 80's.As i remember the intake bolted straight down like magnum intakes and without mods would not use other small block intakes,unless they changed by 68?If its the original intake is the egr passages plugged up?If so that will lean out the mixture and make it ping...If thats okay then i would guess that the carb which i believe you posted is not the original..It probbally needs to be getted up as its just flat out running to lean..The oem carbs are calibrated to the amount of exhaust gas they design to be put into the intake mixture by the EGR valve or jets in the floor of the intake..If they are plugged it is going to lean it out,keep that in mind..m880