My big worry is that you're going to spend thousands of bucks only to be denied access to the track when you get there and the chief of tech says "You can't race a truck, they'll roll over!"
I like people building "quirky offbeat stuff", but I'd hate to see someone make the wrong decision and not be able to enjoy what you've built. I would suggest sticking with a more mainstream platform.
And then there's the money thing.. Racing ain't cheap, and neither is building a car.
If you build the cage yourself, you're looking at $1000 or so. If you pay a shop to do it, double that. Mine was $3500 by the time I was done, and it's a pretty basic cage.
Then you have to get the truck to and from the track. it's no longer street legal. If it is, you shouldn't be driving it on the street with a roll cage and no helmet. so add a few more thousand for a second truck and a trailer. don't forget to insure, fuel, store, and maintain them.
throw in entry fees and everything else, and "a weekend at the track with my cheap race car" turns into $1000+ after entry fees, food, lodging, gas, and maintenance on the hauler.
I'm really not trying to be a downer- I promise. just trying to be realistic.
I built my own car (all except the cage) from a daily driver, and it's cost me about 3x what I could buy the same thing for. And then it costs my another small fortune to race it!
I did it because I wanted to build a car from scratch and I applaud anyone who wants to- just be realistic about what you're doing and realize it's not goign to be nearly as easy or cheap as you think you can do it. then throw in the curmudgeons at the track who don't want to see a truck out there racing against his brothers 1960s british car with a 0.025L engine that revs to 20,000rpm and then catches fire.