Hi!
Hope you can stand my sometimes bad english.....
I have a Kadett B Rallye Coupe 1969....
There is a number of alternatives if you like to upgrade your brakes.
I don't know how common opel are at your place but here there's a lot of them....
You think you still have a firm called "Splendid Parts (US)"....they are very good at the GT and the Kadett B is pretty much the same.
I use a kit from the same firm (Germany) that makes you able to use the 13" wheels, bolt on.
http://www.splendidparts.de
For registration here (with a lot of horsepower)in Sweden you just need the brakesystem from rekord 2,2 (vent discs).
You can also look at the forum
opeltuners to get some new ideas.
Please feel free to contact me if there is something you might need from Sweden.....
Regards Beginner