better top end and better fuel mileage

I was wondering if anybody had any experince or input in the good ol reliable 4.3 v6, I just purchased a GMC Sierra (SL basic model)4x4, a month ago with a newer engine and clutch it has been running fine. I was wondering what one would go with for aftermarket parts for getting better fuel mileage, also when I shift it into fifth it takes for ever to get from 90km to 120km.  what kind of after market parts would i go with to bump up the top end preformance? I was told by a mechanic that i can go as fast as i want in fourth without hurting the engine or clutch then when i hit the speed i want shift it into 5th gear, because fith is suposedly overdrive he says. the only concern i have is my shift indicator lights up when i am in 4th and start to go past 80km. I get kind of weary thinking Im over reving the 4th gear when this happens ? 

please all input would be greatly appreciated.

The shift indicator is to help you save gas milage not a true shift indicator where you must shift.