From my experence the easiest thing to do is to start from scratch using photos, there seems to be a distinct lack of CAD drawings out there.
Although I will add that it is worth contacting companies directly. The teams themselves will be unwilling to release plans but I have had some success with other manufacturers... Hewland were most helpful, and Cosworth seemed quite willing to help with details of their DFV/R engines.
Other than that there are a couple of sites with 'Plans' available in raster form....
http://www.suurland.com/
http://www.smcars.net./
My only other suggestion are model plans, they seem reasonably acurate.
Unfortunately these do intail a bit of work to get them into CAD form.