[FWB]

I don't know, maybe i'm reading into the problem differently, i don't think it's a carb issue at all, a surge or buck upon accelerating i'd be with you on carb isn't set up, but the description is decelerating and downshifting, i would be looking at your brakes hanging up or a driveline issue. did the car sit for a while before the new carb was put on? could the old carb have been so bad you didn't notice the other drivability problem?

if there was a lean condition upon decel, there most certainly would be a popping out the exhaust or a pop when shifting......