GMC53;
It still sounds to me like you aren't getting enough fuel to your carb. Are you sure you have enough gas in the tank for the fuel pump to pick up? If you have at least 5 gallons in the tank,try pulling the gas cap off,loosen your fuel line and see if you get fuel flow. If so,try to put a clean bucket under the fitting you loosened and open the fitting. If the gas is flowing well,re-assemble the connection. If there's no gas or a very weak stream,coming from the gas tank,attach a piece of fuel hose and apply a little air pressure to the line going into the tank-just a short burst of maybe 5 pounds of air pressure. Does it flow now? If so,you might want to clean the tank out,and now would be the time to replace your sender if it isn't working,or at least the screen on the end of your fuel pick up tube in the tank.
If you had a good fuel flow from the tank and didn't have to go through all this so far,let's go back to the electric fuel pump. If it's wired right,you should hear it running for a few moments every time you turn your switch on. If not,loosen the the fuel line fitting at the carb and turn the switch on. Now you should hear it running,and gas should be coming from the loosened fitting. If you hear the pump running but there's no gas showing,have a second look at the pump and be sure it's installed the right direction. (this one caught me once-don't ask how) If that's right,is the line from the electric pump feeding into the stock fuel pump? If not,see if there's an inline filter that might be plugged. If it's feeding the stock pump,pull the oil dipstick and smell it;do you smell gasoline or stale fuel smell? If so,your electric pump is pushing gas past a cracked diaphragm in the stock pump and into the crankcase. Bypass the stock pump and see if you have fuel going to your carb. If so,do an oil change,tighten up all your lines and see if it'll run. If not,loosen the line at the carb again,and see if fuel is making it to the carb. If so,you may have to re-check your float level in the carb. Now you've been through pretty much the whole system,so,for all intents and purposes,it should run. One last possibility I just thought of,make sure your gas tank is venting properly,otherwise the fuel pump will pull vacuum against the tank to a point where fuel won't flow until you let it sit a while.(This probably won't apply,since this problem usually causes the truck to run fine for a couple of minutes and progressively get worse and worse until it runs out of gas, Once it sits for a couple of minutes,or you pull the cap to add more gas,it'll be fine for a couple of minutes,then...)
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