Yes, for 1967 the paperwork and the original engine have matching Engine Unit Numbers, not partial VINs. So there is a way to verify.
But in a nutshell, if you have a 350 Pontiac engine in a '67, you do not have your original engine due to the fact you stated above - Pontiac did not install 350's until the 1968 Model year.
But actually the story about the engine replacement makes perfect sense. The car is brought in under warranty and the dealer determined it would easier to just replace the engine. So they go back to Pontiac for a complete new replacement. But Pontiac is now producing the 350 (it's now 1968) and not the 326, so that's what the dealer installs.
In essence you got a factory service replacement engine of a different size. It will be a very interesting car show story if you have the paperwork to back it up.
2012 Mustang Boss 302 #1918, Competition Orange. FGF replacement 2006 Mustang V6 Pony, Vista Blue. Factory ordered. 2019 BMW X3 (Titled to the wife, but I'm always driving it for her. So I'm claiming it) Old projects, gone but not forgotten: 1967 FB 400, original CA car. After 22 years of work, trashed by the guy who was supposed to paint it. I had to sell it. 1980 Turbo Trans Am 1970 Mustang fastback, 351C 4Bbl, auto 1988 Mustang GT, 5 speed 1983 F-150 4x4, built 302 1994 Chevy K2500 HD 4x4, 454 TBI
