Carb Issue or something else?

I recently installed a recently rebuilt Quadrajet on a basically stock newly rebulit 400 SBC. It will crank and idle and run fine until I deccelerate when going down the road, at which time it seems like it runs out of gas. The motor starts spitting and sputtering and will die if I don't press on the gas pedal. Even when moving at 0 mph, as in pulling in and out of the shop if i stomp on the brake it will almost die. I am thinking that I may be losing vaccum some how but i don't have power brakes or any thing like that. OR maybe the gas is going some where???? because of the slowing of the vehicle.The motor is basically stock with .488 lift .284 duration crane cam, and a Air Gap Intake. Any Ideas?

Could be a leaky fuel pump diaphram,,,when your engine speed is up,,it may pump enough,,but when your RPM's drop it may not be able to pump more than it is bypassing.

Also what is your idle rpm set up at??
A lumpy cam will not create a lot of vaccume,,and engine idle may have to be increased.
Just guessing.