Fuel gage for some reason my fuel gage isnt working, it stays at full all the time. what would the problem be?

Check the work. The fuel level wiring might not have gotten plugged in. Also, if the fuel pump was a pain to get out for whatever reason, the wiring on the inside might have gotten broken off or the arm that holds the float got bent. Lastly, it doesn't happen a lot anymore but sometimes the float just sinks and stays there. The newer floats are made of foam so they tend to last.

I didn't see an s15 in that model year, in my database?
If the speedometer works but the tach doesn't, I think something is wrong with instrument cluster?
GM has had a problem with fuel gage sending units. The sending units are not hard wired to instrument cluster, they send signal to pcm, the pcm sends serial data to instrument cluster, telling the cluster the gage settings.
It may be possible to use different size resistors in place of the fuel level sending unit, to see if fuel gage reacts properly to different resistors, if the gage reacts properly, then I think the problem is the fuel gage sending unit.

that is normal--it will come down as you use the fuel in the tank

Only thing I can tell, with all you've checked, is the power to the gage isn't there. If the tank sending unit ground to frame is not connected, the gage will go past the full mark. No power to the gage it will stay at empty side. Power goes from fuse to gage, gage to sending unit and back to gage. Make sure the fuse is good on both sides of it. The gage just isn't getting power from the fuse.

Could be the sending unit in the tank, or the gauge on the dash. A mechanic would check the signal from the sending unit at the tank.

PROBABLY YOUR FUEL PUMP,OR SENDING UNIT SINCE THE GAGE ISNT WORKING.CHECK YOUR PSI. ON YOUR FUEL RAIL.

SOUND LIKE FAULTY FUEL LEVEL SENDING UNIT IN FUEL TANK ON FUEL PUMP ASSEMBLY.

bad ground on tank

Try disconnecting the battery for a minute. If that doesn't straighten it out, you may have to take it apart again. The float might be hung up on something. It may shake itself straight next time you put gas in, too.

You will need to remove the fuel tank from the truck and replace the fuel pump/fuel level sensor module that is inside the fuel tank. Use extreme caution when doing this as there is a real fire hazard.