The master cylinders used on the pedal box come in different sizes. Depending on what size brakes you fit, you use a different size M/C for it. You also have issues of brake balance that need to be worked out. To get your car to pass the tests for ADR31 (read it here..
http://www.infrastructure...tor/design/pdf/31.pdf) the car needs to be tested with the front circuit failed, the rear circuit failed and the booster failed.
The advice I got from my engineer when I asked him about modifying the brakes on a 1978 build car was to buy a car built before 1/77. It was cheaper to buy another shell than it would have been to use the engineers time for just this one test needed to get the car road registerable.
Same went for modifying the engine. It is easier to get a complete late model engine engineered in a car built after June 76 than it is to legally fit twin webers to the engine.
Problem with fitting a later model engine to the car is that you also have to make other safety upgrades to the car at the same time.Retractable rear seatbelts was one of the things. It all depends on what the engineer wants.
Just be prepared to open your wallet and remove the contents cause it'll always cost way more than you think.