hey neil, several months ago i bought an fc sedan with an hr disc brake front end and a commodore disc brake diff already fitted, however the car was fitted with a booster and master cylinder from a v8 hk holden. ( the booster is huge) i have since switched to a booster and dual circut master cylinder from a vh commodore (4 wheel disc) the booster is alot smaller in size and will mount to your firewall but it seems the travel from the pedal to the booster on the commodore is about 10mm shorter than the hk so either build a new bracket, or as i am doing cut the commodore booster- firewall bracket down 10 mm shorter. and weld a new plate on the end.
and re-drill the holes. (easy)
also the mounting bolts on the booster for the master cylinder are located diagonally, these are the only hiccups i can see but the car is still several months away from driving...
this is all new to me, first rebuild 18 years.....
hope it is some help
dean