If you want to compete in stock class your wheelbase can't be modified more than 3 inches. That's the rule that makes a lot of rigs not eligible for stock. Depending one who's comp you are competing in sometimes(oldschool rules) you can't change your suspension design, if your rig came with leafs if has to remain leaf sprung. Werocks rules state that you can add coil suspension but your tub can't be altered to fit it. Other than that for oldschool rules tire size limit is 37 inch and werock's limit is 35s. Both require a complete body, un narrowed and factory bed, minor fender trimming is allowed.
As far as the next class up, werock calls it pro mod with 37 inch tires, no wheel base requirements. 37 inch tire max. Must resemble a vehicle, same body lines, and bed sides. No rear steer. And has to have 2 or more seats. Old school calls it the legends; they call for 40 inch tires has to have a box frame(I think that should be better I forced this year). No rear steer. 2 or more seats.
As far as unlimited goes, I think it's pretty much unlimited but I've never competed in this class so I'm unsure of the rules.
All vehicles in werock and the legends and up in old school. Require a 6 point cage and harnesses, bladder fuel cells, kill swithes. Everyone requires 2 fire extinguishers and dot helmets. Werock requires a firesuit, window nets, and a winch with synthetic line. The stock class is also required to have harnesses and 6 point cage as well.
That's the basics. There a ton more technical stuff but you get the idea
