If I were going to go to all that trouble I'd use new materials for the job. Besides, the sprues will probably be of larger diameter than what you need and then there is all that trimming. If there is a decent hobby shop in your area they should have a stock of Evergreen or Plastruct styrene in all sorts of sizes and shapes. Just about anything you could imagine. Especially if they sale railroading stuff. There's also K&S tubing and sheet in aluminum, brass, copper, tin. None of it is very expensive, particularly the styrene. All the manufacturers have websites so you could check it out online.
If you could find a copy, Car Modeler 2002 has a detailed article on scratchbuilding a roll cage for a stock car and may help with the brainstorming part of the project. There's very detailed instructions and the author goes as far as showing you how to build the jigs he used.
If you haven't completed your project yet and can't find the mentioned mag, send me an email.