74 Duster 318 with 904 Trans

My guess is that at idle, the Transfer slots are inactive/or nearly so, and the mixture screws are cranked rich.
Plumb the VA to the sparkport,
Reset Idle-timing to 10*, and
mixture screws to, in the center of their range, (~2.5 turns out on just about anything besides a Holley), and
Then increase the rpm to whatever doesn't bang the trans on the N>D shift, but also eliminates the Tip-in Sag.
This will be your starting point.