Any carb on a blower without a few mods will act this way. A carb as we know it senses vac. and you just bolted it on a huge vac pump !! Carb will think it's running at full vac all the time. Surging is normal depending on cam size and drive ratio but can be reuced to an OK point
The edlebrock is a knock off of a Carter and it is one of the most adjustable blower carbs around. You will need to find the proper jet size but start with these mods to get you to point of fine tuning. Stager or delay opening of second carb by a few hundred RPM to make more street friendly.
Remove the needle (16) by swinging plate to the side, and take a measurement of needle in its installed up height. Now install a metal rod/pin inside (15) to keep it in up position. If Vac. overcomes the spring it pulls down the needle and reduces fuel by plugging jet hole with metering rod. This is not good for a blower setup so you need to disable it.
29 and 31 have holes along the tube located below gasket surface in photo. These holes emulsify or add air to fuel being pulled into discharge nozzles. If you solder top hole closed it fattens up carb big time so go slow, memory is about 4 holes on each tube.
Remember no vac advance allowed on distributor either so lock it down.