What kind of Idle are you running at stop signs?

If you change from a poorly operating carburetor to a good one, that will definitely smooth out your idle in gear. My 69 slant idled terribly for decades. Turns out all the rebuilt Holley 1920 carbs I tried must have been bad. I finally got a good rebuild and it purred like a kitten at idle. They have a sealed idle metering block that gets clogged up and most rebuilders don't address that. That causes it to idle lean, which also makes it touchy to changes in load, like N vs D. All things equal, a 1 good bbl should idle better than a good 4 bbl since the later is a bit over-fueled. For now, try increasing the idle spark advance to at least 10 deg BTDC and might run better. If you don't have a timing light, just turn the distributor until it it idles the best and then watch for pinging going up hills to tell if you went overboard, and pick up a timing light for $2 at a garage sale.