The 70 383 Mag in my Power Wagon did the same thing. Eddy 600 AFB, larger, high 400's cam & not much torque to get the beast rolling.
I added a 1" four hole phenolic spacer & that helped the carb signal, but it would still pop off idle at a light if I wasn't careful. Leak down test revealed two dead cylinders & my hands would get wet with fuel when I held them over the tail pipes. I suspected a blown head gasket between cylinders, but when disassembled to install new heads & smaller cam, turns out the orig heads did not have hardened seats & the exhaust valves were sunk into the head so far, they weren't sealing at all, allowing the intake charge to go right out the exhaust & causing a lean condition. New heads with hardened seats, smaller torque cam & viola, no more popping thrut he carb. The other guys like Randy, who're a lot more mechanic savy than me, might chime in about burnt valves causing the popping.