1990 Gmc topkick with 366 motor running rough, wont stay going

Try this website. The fuse panel is in the dash near the steering wheel, probably at about knee height. Some versions have the cover held in place with small bolts. (possibly 8mm or 10mm).

https://www.autogenius.info/gmc-c-series-mk2-second-generation-1990-1999-fuse-box-diagram/

I'm not sure the issue...if you think it has 3 mounting bolts or stud/nuts instead of two on starter, and it may do, 3 bolt starter bolt patterns are not unknown, then...have you undone any bolt which might be holding the starter on?
btw, best disconnect battery earth lead before disconnecting starter, or else main cable on back of starter is live +ve and exposed and will go ZAP!!! if spanner or watchband arcs across it to engine block as you work around it.

https://www.mylittlesalesman.com/1992-gmc-366-427-alternator-8977307

Replace filters and flush lines

GMC

to get your solution visit this link

Before going there, check your INERTIA SWITCH (INTERTIAL CUTOFF SWITCH). A small accident or a sharp bump can cause the cutoff switch to trip. The switch is located adjacent to the passenger side rear shock tower, and there is a pushbutton to reset (on most Topkick tucks). Carbureted Topkick trucks do not have cutoff switches, and some custom trucks have the inertia switch relocated.

Electrical problem. Something gets hot and stops working. Could be fuel pump or something in the ignition system, so losing spark. After it stalls, spray starting fluid in the intake and try to start. If it fires up and runs a few seconds, you know it's a gas problem. If starting fluid doesn't do anything, check for spark, it's probably an ignition failure-possibly the coil or the ignition control module. The module can be tested at a parts store.

Someone please call me 3365179318 I have a 1992 gmc topkick 366 engine and I noticed when I turn the key the fuel pump is not coming on. So I’m trying to find the fuse or relay for it to see if it’s maybe bad because the fuel pump ain’t even a year old. Someone please help me lol

check the engine temperature sensor.

it is possible that you have transposed the wires to the starter pos to neg.