I think stock big body disc brakes came in on the 67 and 68'* so that may be a bolt on swap but, generally they were just better ... not great.
I put the (factory) aluminum 8-lugs on my 65 gp and they have little fade etc, cause the finned drums are open to the air.
If the car is a real keeper (and looker) they are remanufacturing these (Arrowhead Components
) (scroll down a ways on the site).
... And they look great (Maui Auctions Listing Photos
) if you love the mid 60'* look as I do.
(Maybe even the GP parts might work since all of the cars were "big" ...although my 69 gp is considered "extended A body" ...?)
2005 Bonneville GXP – Dk Grey
2001 Grand Prix GTP
1994 Bonneville SSEi – * Port Magnuson SC
1969 Grand Prix SJ – 428cid/370hp