On the front of the carburetor is the accelerator pump. It's a lever that moves when you kick the throttle. It squirts a shot of gas into the engine to go with the big shot of air that comes rushing in when you open the throttle.
The amount of gas you need depends on a lot of things so they make the accelerator pump adjustable. There's a screw at the end of the lever that you can turn to make it shoot more or less gas. There's also some pieces that change the geometry of the lever to kick in more or less gas, as well as a "jet" (I believe they call it a nozzle or something like that) to meter the fuel.
Start with adjusting the screw in or out. You have to take the truck for a spin to test the full effect. Just revving it in neutral in the driveway won't cut it.
Just adjust the screw a little, go for a spin around the block, adjust it again, etc. until you get it to stop stumbling.
If that doesn't fix it, your best bet is to swap out the secondary springs (inside the black plastic part on the back passenger side). That controls when the secondaries open. Get the full spring kit from holley so you can try them all out. Then start swapping them until you get the right one in there. But that's a good thing to do anyway on top of adjusting the accelerator pump.