Does anybody know what parts from GT i need to put the front and rear disks from gt to 75? e.g. what do i need to change on my 75 so i can put disks on from gt (front & rear)

Firstly I wouldn't bother with the GT front brakes, they arn't great, for the money you'd be better off getting some after market grooved discs. They start at about £50 a pair, they will be better and you won't need to start spacing the calipers if your ones are smaller.
The rear discs are easy, all you need is the trailing arms and everything attached to it, arm, hub, disc, caliper etc. This just bolts in place of your original one and the foot and handbrake connected. (same connections as yours).

Find a breaker with a front shunt GT (shouldn't be hard).

Thanx Evil-Punto
but can i put in front instead of standard disks selfventilated ones? Is there enough space?

Selfventilated = drilled, yes no problem. If they are designed for your car then the will fit OK.