Hi
I just found this post while looking for something else, I hope its not to late to help you. I have a 73 1300 Bug, it has a stock balljoint front end in it. I'm running 6x15 Cookies, these have a 36ET and fit nice. So if you add your 18 + 36 you would need a 54 ET wheel in a 6 inch to fit nicely, in a 7 inch wheel you would need a 67 ET, something your unlikely to find. So I think you need to ditch the adaptors and get you drums or discs re drilled.
You can have a play around with wheel ET on this page
http://www.1010tires.com/WheelOffsetCalculator.asp
Steve