I’ve had several of those trucks with the 350. Does the check engine light come on, and do you know how to retrieve the codes stored in the computer? Really easy to get the codes on an OBD1 system GM with a paper clip.
As for your exact problem, could be several things. Change the fuel filter if you haven’t recently because it’s external to the tank and routine maintenance anyway. If that’s no help, and assuming there isn’t a code set pointing to a particular problem, you might have the fuel pressure checked to see if all is well with the fuel delivery. You can also remove the air filter and breather assembly and have a look at the fuel spray pattern in the throttle body. Compare your spray pattern to pictures of proper spray patterns online. I’ve seen example pics years ago, but would have to search to find them now. Good luck. I’ve got a soft spot in my heart for the TBI 350’s and probably always will.