I need a little help on this subject.

I purchased my 1968 firebird convertible in 1968. The car was tagged on the hood as a 350 ... and I subsequently verified that it was manufactured as a 350. I was informed at the time of purchase that the current motor was a pontiac 400. From the engine serial number I determined that it was a 1969 GTO 400. Until recently I assumed that the motor had been replaced at some point. However a few months ago I stumbled upon some information indicating that there was an option available to have a 400CI motor installed in a "non-firebird 400" vehicle. Does anyone have any knowledge of this? I'm reluctantly considering selling my FB and am curious if it's possible that my car may have been one of the 350s upgraded to a GTO 400 motor. Any thoughts?