I have the book too, and concluded that it was false at the time after a quick check at the local Pontiac dealer - no Chev parts for "Canadian" Firebirds were listed in their parts catalog, which I also have a copy of. Some of the guys had been around at that time and were only to glad to educate me on the subject.
The excerpt in the book is just another repeat of the con perpetrated so Pontiac racers could run in the Trans Am series, not the beginning of the myth. That was started by the racers mentioned. I sent a polite inquiry to Milt Minter racing regarding replicating one of the "Canadian" Firebirds - no reply there either!
Most Canadians knew that there were differences between US built and Canadian built cars, including Chev drivetrains in most of the Pontiacs. Except the musclecars: GTOs and Firebirds received "the Blue Block", "the American Big Block", "the US Pontiac engine" as they were known to the few people up here who had or knew about them. As to how many actually heard of the myth, it would have been a very small number, and even fewer people would be at all concerned. I'm apparently the only one putting up a stink about it! But all the officials had to do at the time to quash the rumour was to call up a few dealershups at the time and ask.
My contention is that the officials went along with it in order to get the car count up. In their eyes, the Camaro and the Firebird were the same cars, just like the Mustang and Cougar, and Challenger and 'Cuda.
Good scam indeed! And by the way legends are the stories that usually have a germ of truth, not myths! Was Not True!
I guess I'll have to start talking about Big Block Pontiacs, as presented at the top of the cover of the recent Pontiac Enthusiast magazine! They should know, right? Must be true, it's in print!