Double pumper vs single feed

Whether a carb is a single feed or dual feed has nothing to do with whether it has mechanical secondaries (double pumper) or vacuum secondaries.

Most of the single feed carbs have vacuum secondaries and you can't convert one to a double pumper without changing the throttle plate and main body. Those parts new would cost nearly as much as a new carb. If you were to get those parts used they would most likely come as part of an entire used carb.

Holley sells kits to convert a single feed to dual feed. It is new bowls with center hung floats and gaskets. Won't do anything for performance except if you are into road racing where the center hung floats are less effected by lateral g's.

Double Pumper carbs have a slight advantage over a vacuum secondary for max acceleration at the expense of being harder to tune, being less forgiving regarding what size to use and providing less fuel mileage. Unless you are very good at tuning, your car will run as well or better with the vacuum secondary carb.

If you have a Holley 600 vacuum secondary carb it would be my recommendation to leave it alone and if you need something else go shopping for a differnent carb.