By all means experiment with carb spacers and don't stop at only 1". Try one spacer and then add another to 'see' the difference. This may kill a tad of low end but, though not really familiar with your form of racing, methinks once you hit the pedal your climbing up the rpm range fast and holding it there.
Header wise, yes, go to a 3" collector and if you can, you may want to experiment with a 3.5" collector. That and collector extensions about 2' long. Then use a large yellow/white crayon and mark a thick and parallel line on the side of each extension. After a few runs, you'll see a burn mark as noted in the link's picture. Cut the extension off at the end of the burn make and you've optimized at least the overall collector length.
http://www.headersbyed.com/hc_cheapperfgains.htmCam wise, I'll guess the cam was degreed and installed 'straight up". If so and low end to get going isn't a big issue, you may retard the cam a few degrees (top end gain/low end loss) and see what happens. Lotsa work but pretty cheap with a multi-indexed camshaft gear set.
There is hp to be gained (or lost) based upon your fan type:
http://www.network54.com/Forum/215655/message/1066184641/Car+Craft+Cooling+Fan+Dyno+TestThe best would be electric fans followed by an OEM type standard duty (not HD) thermal clutch fan.
Because the truck is kinda' stationary when moving (unlike a drag or track car) you could for a few $ weld up a deeper oil pan
from two used factory pieces (and a longer pickup tube) to minimize splash and likely get a few more ponies. Most cars don't have the room but I'll bet you do. It's possible though this move may not gain any free hp since the oil isn't piling up in the back like it would in a high G drag car.
One other thing to try.....racing gas. A dose of better-than-pump-gas in the 108-112 range that is highly oxygenated should provide a few more hp. Btw, one must play with total timing here a bit to see any gain. The goal here isn't to break your budget but to point to another issue, your compression ratio. Likely, a gain in power with racing fuel would indicate, if you can afford the gas, a need for more c/r. Sounds like we'd all know this already but even small C/R increases (say milled heads) from your current 10.5 to 1 to a higher number would outstrip the smaller gains IMO from spacers, larger collectors and optimized collector lengths.