It's been a while since I played with a holley.. but here goes.
Anyone can correct me if I am wrong or left anything out.
Holleys have an external float adjustment and a plug with a ***** on the side of the bowl.
The adjuster is on top and should be a ***** with a locking nut.
First, have some rags to collect any spilled fuel and if possible, a fire extingisher.
Start the engine and remove the plug on the side of the bowl and determine if any fuel comes out, or does not come out. (I used to tap the bowl lightly with a ***** driver handle to make sure the float isn't sticking also)
What you want is for the fuel to come up to the bottom of that hole.
If the fuel level is too high, a lot of fuel will come out.
If too low, then no fuel will come out or be seen.
Too high and the float needs to be lowered with the adjustment *****.
If too low, then the float needs to be raised.
When you get the fuel level where you want it, hold the adjustment ***** so it does not change, while tightening its locking nut. Then replace the plug.
It is also possible that the carb has junk (dirt or varnished fuel) in it that is interfering with proper fuel delivery.
Do you know if the truck was driven often or parked for a long time before you got it?
Also, did you look at all the vacuum lines and blind plugs for wear (wear or hardness, like the rubber has lost its elasticity), dry rot, rub marks or obvious leaks?
Kinked fuel lines, leaks etc along the fuel lines?