This isn't complex. No mysteries. It is important to remember that Canadian Valiants are actually Dodges though they were branded as Plymouths. Dodge Wheelbase. Only the front sheet metal (fenders/hood) are Plymouth Valiant. Firewall back a "Vodge".
I don't know why this was done other than the fact that '64-'66 American Valiants had Barracuda back ends. I don't believe Barracudas were mfg in Canada. Thus Chrysler Canada made Plymouths out of Dodges. Dodge sheet metal and what else? If in doubt about anything referencing an American FSM could be counter productive. Likely a Dodge FSM or Canadian (only) Plymouth FSM is the choice if you don't have one.
One has to careful when ordering parts. As an example I had an issue with TTI. I have 106" wheelbase American Valiant. I received $800 complete bolt on 2 1/2" exhaust system for a 111" wheelbase Canadian "Valiant"! :BangHead: