I did not see what heads you have right now, but I will guess the stocker 76cc 1.94/1.5's. From what I see, the cam/head combo is what is holding you back. For a heavy truck, mid range power is what you want to achieve, and an effective way to do this is to run a set of late model vortec heads and a larger, under .500 lift cam.
On the cam, stick with a hydro, around the .460-480 range and don't go over 220 degrees at .050. You will also need a new intake manifold, Edelbrock super victor has had positive results on the vortecs. This will really wake up the engine (400hp has been seen), and possiblly give you a boost in mpg, believe it or not.
The investment should not be more than 8-900 bucks with just the purchase of the new heads, manifold and new cam. Of course this is doing all of the work yourself, the cost goes up dramatically with a pro doing the parts swap.
Do a search on this forum,
www.chevytalk.com and
www.teamchevelle.com for more insight on cam/manifold selection.
Hope this helped.