help me calculate fenderwith

[tom'72]

here's the deal, I'm running stock type 1 discs up front , porsche carrera discs in the rear on a standard automatic chassis.

wheel dimensions: front=18x35x8,5 ET56
rear= 18x35x 10 ET50 (come from a Porsche GT3)

I'm putting together a chassis with all of this, up front I estimate I need 30mm spacers on each wheel to clear the suspension and that would mean front fenders that are about 6 cm (+/-2.3") wider fenders.

In the rear I need 2cm (+/- 3/4") spacer on each wheel to clear the torsion plate.
I'm not sure but I think this way the rear wheels would need only very little wider then stock so my only option would be to get the front beam shortened.

Am I thinking in the right direction here or completely off?
What are your thoughts?