You are correct...this service will be operated with Boeing 767-300ERs.
However, I would be willing to bet serious money that you'll be seeing different Boeing 767-300ERs on this route than are usually seen (ie, those with ExecutiveFirst).
I was taking a tour of a maintenance building at Air Canada in YUL
a year ago, and we went inside a 767-300, which had eight main
exit doors (not four doors with four overwing hatches). This aircraft had Air Canada's usual domestic-767 ExecutiveClass seats installed.
Does anyone know the origin of these birds, because I'm almost certain that these are the -300ERs plying the "SuperComfort" routes (alongside the ex-domestic-200ERs serving "SuperComfort"). I don't think they're ex-CP, but perhaps they are.