i'm not sure how many people involved in building or maintaing race cars will actually rattle through here, but i'm interested in sharing knowledge and tips. in addition to having built an SCCA IT sedan, i've been an SCCA tech inspector for more than 10 years and have seen a couple of race cars.

I have not yet built any racecars (or even a souped up car), however that is something which I have always wanted to do. Just never had enough time and/or money. As I have a small fleet of VW Rabbits, and one of them has a severely rusted body, I have hit upon the idea of building a Formula VW in the style of those I used to see when I was young, but using the more modern drivetrain. Hopefully it will fit into the SCCA Modified class. Even if not, I figure to have lots of fun driving it as I intend to keep it street-legal!

you can run most anything you want in Mod in SCCA Solo II (autocross), so long as it meets basic safety standards. Road racing is harder, there are a lot of fairly complex rules to navigate. as a tech inspector, i'm more than happy to help guide you through them.
your VW driveline might be useful in certain formula cars, or in a tube framed GT car. is it a 1600 (i'm assuming watercooled), or newer (and larger)? the early watercooled 1600s were used in the second generation Formula SuperVees, which are still classed as Formula Altantics, although they are rarely seen anymore. you could look at the newish Formula S which is for "specials" that don't fit naturally anywhere else. be aware that whatever you design has to pass muster in the club racing tech office in topeka; SCCA cars at places like Watkins Glen and Pocono get going fast enough that you really want an experienced engineer's eyeballs on the design at some point in time.
for a full fendered car, there are numerous spots where VW products are classed, although a rusty shell isn't an ideal starting point for a tub-based class (Production or IT).