I chime in a little late for help, but start with the easy simple stuff first. Do a visual of the fuel pump fuse, relay and the connector at the pump under the bed. A restriction at the fuel filter will over work the pump resulting in higher current. 7.5 amp while running should be normal and many accurate amp-clamps are reasonable, just place over the power wire to the pump, or fused jumper wire install at the fuse. High current will cause terminals to look burn't or brown in color.
Using a digital multi meter, check fuel pump fuse on both sides with engine running or the two seconds the pump is commanded on at key-on. You are looking for a low voltage drop across the fuse or relay's terminals 30 & 87 if you use 4 jumper wires to operate the relay outside the fuse/relay center. A low voltage reading is well below 1 volt, perhaps 0.05 vdc in a good world.
Fuel leaks past poppet type injectors if fuel gums up on the end, cooked after shut off from engine heat. Cleaning these injectors require a pressure pot with the return blocked off. 60 PSI is used with ADCelco X66P or GM upper engine lube at 20% and the rest gas. If a pressure pot is not handy, Techron by Chevron is ok for use in the tank as it will not damage an electric fuel pump. X66P & GM upper engine lube will.
Fuel is captured returning on the supply side by the fuel pumps check-valve built into the pumps outlet. The regulator prevents leakage on the return. If both are blocked off, the leak has to be injectors, o-rings, or housing (Loose fitting)
Volume has not changed regardless of EFI or carb, so 1/2 Pint in 30 seconds is normal. Use the "Prime" terminal next to the fuel pump relay, supply B+ thru fused jumper wire to check.
Your 98 C1500 5.0L uses a bottom referencing fuel pump bucket type w / EVAP pressure sensor. This type of pump is serviced with the bed removed or lifted high enough to replace. This pump assembly has a sock type filter inside on the pump and one on the bottom of the plastic buckets inlet. The bucket design is to hold fuel around the pump will no air bubbles, but cools the pump and lubricates it. Yes, this means is it is run on "E" most of the time, the pump will fail quickly. If the pump comes out, disassemble the plastic bucket (3 tabs / pocket screwdriver) and inspect bottom filter and the inside of the bucket. If there is a lot of dirt, dust, rust or whatever, you must pull the tank, empty properly, put the hottest water you can get into the tank, then agitate vigorously, drain, wipe dry and place where low humidity will help. (This procedure is in GM service information) A new pump will quickly fail if tank is dirty over years of re-fueling events. If returned under warranty, all are inspected by a field rep and the claim is rejected.
Tools, DMM, jumper wires, cleaner and working using best practices will help you identify the root cause of you problem. Poppet injectors require fuel pressure between 56 - 60 PSI to pop open, then close as pressure drops. If it has not be upgraded to MFI electric injector set (Second Design) and the leak down leads to injectors, $250 with some gaskets will fix this issue. The problem is, if a poppet injector is stuck shut, it cannot be be cleaned with chemicals. Also, Berrymans; Chemtool B-12 is recommend. Some will argue, but "Sea Foam" is a loss of money. Best of luck. ASE Master since 78 / Retired
