Since you're seeing this during heavy acceleration, have you checked the throttle kick down adjustment? It's the sliding rod which couples the throttle mechanism to the transmission. It's notorious for causing such issues. It adjusts fluid pressure in the transmission, so it's a big deal.
Do you have a service manual? The adjustment is described in the section on transmissions. I could dig up the adjustment procedure if you don't have it.
Also, the kick down rod should be spring loaded at both ends; be sure the springs aren't missing.
While you're in that area, before making the kick down adjustment, make sure that when you push the accelerator pedal to the flood that the butterflies are actually opening fully. If not, you can adjust the opening via the threaded rod which couples the accelerator pedal to the throttle mechanism on the carb.
-It's been such a LONG TIME... BlueShift>>
1981 Dodge Ram B250 Custom Sportsman Maxi Van
It's what you learn after you know it all, that counts...
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Learning from my own mistakes is good, learning from yours would be much better!