They didn't produce any factory 428 Firebirds or GTO's, but yes, some dealers did swaps.

Motor Trend did a feature article on a 1968 Royal Bobcat 428 GTO, and I believe the cost was around $600 for the swap, plus of course they would get your original 400. Not a bad deal really. I'd imagine if Pontiac offered a 428 as an factory option, it would have been around $150-$250.

There was a 67 or 68 Firebird in a Pontiac Enthusiast or High Perf Pontiac magazine a LONG time that was supposedly a Royal Bobcat 428 Firebird, with dual quads.

Myrtle Motors also did a conversion, and tried to get in on the Royal Pontiac publicity wagon by having one of their 428 Firebirds featured in the Sept 68 issue of Popular Hot Rodding Magazine (attachments below).

Either way, there weren't any factory 67-69 428 Firebirds or GTO's, and there won't be any PHS invoices or builds sheets showing a 428. If this gentleman has one of the dealer conversion cars, it would be worth a lot of money restored today, as long as he has a rock solid paper trail to prove its heritage.

Here's a rough scan of the 1968 Myrtle Motors Firebird 428 test. This baby flew.