I have a little bit of different advice from the rest, but I think the first thing you should do is gears. That truck was probably highway geared from the factory, potentially even 3.23's. Assuming you have a overdrive transmission in 1990 I'd swap the gears to a 4.56 front and rear and you will see a very dramatic difference in performance. Deep gears, opening the intake and exhaust up a little and it will probably surprise you.

If you want to do it on the cheap, its easy to find 4.10 geared D44 fronts and D60 rears from W200 trucks in the junkyards.

-Jon