I was a Tool & Die maker for 20 years. Made progressive dies more than large single hit dies. The time and cost to build a die means you typically expect high production, not one or two offs. How many of the same car bodies are trying to make?
First obvious question is the press itself: what is the size of the press bed, the shut height, vertical stroke of the press, and speed of the stroke (typically strokes per minute)? Is that big enough for your pedal car body (also allowing for the form blocks outside the final shape)? When I picture a pedal car, I am guessing 18" wide by 24-30" long. Trying to stamp that in one hit with 12 ga is going to be a challenge for 300 tons, but depends how deep you need to go and how well defined form lines you expect. By breaking the body into 2-4 sections, you have a much better chance of success.
And yes, as mentioned, 12 ga is .105 nominal thickness, so very heavy for a pedal car body. Even full size automotive body panels back in the 40's were less than .042" thick.
First thing you need to learn is how to unfold your intended shape so you know what size your flat blank is. You can find information on this online. Complex curves are much harder to unfold than rectangular shapes.
You really do not need draft for metal stamping, but deeper draws do tend to spring open a half degree or more. Your struggle is going to be buying consistent thickness material if you want to make several, as cheaper sheet stock tolerances for 18" wide will be +- .005. Doesn't sound like much, but if you built your form clearance for exactly .105 material and you put .110 material in it you will blow your forms blocks out of position. If you design for .110 material and you put .100 into it, your forms will be all washed out and have very large spring back.
For large single hit dies you have a few design considerations. Simplest is if you can make a punch matching the inside of the body, then press that into a hard rubber or urethane lined die. Will be cheap, but won't have good form definition. If you build a solid steel form die, you could cut blank to size first manually (or in a blank die) or cut to size after forming manually (or even build a pinch trim die).
Die sets can be bought to custom dimensions to match your press.
