Looking at picking up a 99 Silverado. the fuel gauge doesn't work it's been owned by my uncle since 2002 and the fuel gauge has been not working for the last few years. he say's it's not a big deal if you "keep gas in it" i'd like to know if it is something in the fuel pump or the float itself? and is this a pretty big job (drop the tank/pull the bed off?)

thanks.