74 Duster 318 with 904 Trans

Could also be an initial timing issue.
Assuming a relatively mild cam, and a vacuum advance is present;
Disconnect the vacuum advance and plug the port. Advance the timing until the engine sounds happiest, probably between 10 and 15 degrees before TDC. Adjust your idle back down to around 800 rpm if you can, or close as you can get. Recheck timing. Might have to go back and forth a few times.
Then reconnect the vacuum advance and try it out.
PS- make sure your vacuum advance is connected to ported vacuum, not manifold vacuum.