The other thing you can do it buy a rulebook from the NTPA (
NTPA Rulebook - Publications - NTPA Pull), it discusses many aspects of a pulling sled and can give you some guidelines and is good as it is a standard (something you want if the sport is to grow in your area). It's $6 and $2 shipping (in the continental US), and it's money WELL spent even if you have to pay more in shipping.
I've worked on quite a few and you can "easily" make your own (making it pretty and having it work correctly from the start is a little tougher). A lot will depend on what you have available to use and budget. I can take pictures of our sled and show you how it "works". It's an old international straight framed truck with I beams placed on top (for the weight box to ride on), it uses the stock international axles with power divider (used for moving box via the engine and/or via the axle), and is now powered by an air cooled 5 cylinder Duetz engine and Allison AT540 tranny. To adjust the speed of the box we have two old GM transmissions (one is a SM420 directly mated to another GM 5 speed manual). That said we only use a 3-4 of all possible gear ratios (the others are either too high or too low). Some new sleds use a Profab transmission/gearbox (which does the same thing and even then they can only use certain gears). That combined with a microswitch that controls the dropping of the pan along with weight changes (which we hardly ever change weight) and pan grousers allow us to stop pretty much everything. Our engine/tranny combo is mounted up front between the rails but many newer sleds mount them in the rear and use a pan press to get more of the sled weight on the pan. A lever that disengages the clutch on the box drive is utilized to keep the box from driving through the stops when it's all the way forward is also essential.
I've seen some almost 100% mechanical, some all hydraulic, some with pneumatics, and most have a combination. Off the front of the Duetz we run a heavy duty hydraulic pump (from a trash truck), that powers the hydraulics for the front wheels (lift and steering), and an electric plow pump setup with dump valve for the pan lift/drop. Brakes are controlled via air from a pump mounted on the Duetz and a tanks mounted along the frame.
The ratio of the ring and pinion on the chassis axle, the ring and pinion on the box drive, and transmission ratio(s) of the box drive transmission(s)/gearboxe(s) will dictate the size of sprocket needed for the box drive chains. You want to be able to have the box top out in several gears between 200-300 feet (at least for a sled design such as this). This gives flexibility along with a moveable pan drop switch (microswitch) to stop varying powered/weighted vehicles prior or close to 300 feet. There's a good bit of math involved, but it's not rocket science....
Some places may not have 300 feet for tracks so that has to be calculated in as well in design.
