we just bought a 87 gmc jimmy full size with a nice built 350 in it but the gas gauge dosent work when it says half fuel its empty and sometimes it just bounces back and fourth.
it has a brand new fuel pump but i would like to get the gas gauge to work so what would i need to do to fix this problem

fist off you will need to check the circut. I would unhook the wire at the tank and see if its got current. then have some one watch the gauge as you ground it. if its working the gauge should go to full. most likely its the sender in the tank, but once in awhile you can something else wrong

Had a similar problem, dropped tank and put in a new sending unit. Run a new signal wire too, yours is probably toast by now.

These gauges were notorious for problems in the early 80s (80-~83) the problem is its not a standard gag but part of a PCB DAMN $100 for a fuel gauge is ridicuolous... Whitney used to make an el cheapo $25 kit that included an external gauge