Sorry this is so long getting to you, but I've been out racing with my brother and his dragster.
Anyway, I started my Opel GT project with, and modified, a Jeg's chassis kit. I'm not planning on tearing up the track to the point that I needed a full chromemoly tube chassis with struts. My build is pretty simple, actually. I started by narrowing a Mustang II crossmember which allows me to use narrowed A-Arms and Mustang II rack and pinion steering for my front suspension. I did a 2x3 rectanglular frame and pretty much based it off of the Opel front clip, in order to keep dimensions right. The back half is set up for a four link with a 9" Ford on coil-overs and a track bar/lateral locator. I got to this point in construction using my mockup body to fit all of the frame components and bars for clearance and to tack them in. Finish welds were done with it off the car. The body you see on my car now will be the finished product.
The Hoop and Halo are the only parts that come in the kit already fit to the GT. Everything else is left long so that you can custom fab to meet your needs. Overall, it isn't a bad way to go if you like to do chassis fab. If I were to do it again, I'd probably go through Chris Alston, or S&W Racecars. You will still have the same Hoop/halo issues with them, however.
I hope this has been some help and if you need more questions answered, please don't hesitate to ask.