The 428 engines are not dimensionally any larger than the 400 engine that originally came in your GTO, any header designed for the '68 -'72 Pontiac A-body (GTO, LeMans, Tempest) will work no matter what size Pontiac engine you have in the car. Headers are always a tight fit on these cars, the 428 engine size isn't a factor.
All Pontiac engines of different displacements are the same size externally from the first 287s back in 1955 to the last 455s built in the 1976 model year with the only exceptions being the short-deck 265/301 'economy engines' built up through 1981.
to the PY forums.
Bart