The biggest external difference between a 55 US Pontiac and a 55 Canadian Pontiac is the wheelbase - 122" for US, 115" for Canadian (same as a Chevy). The grille and most of the sheetmetal on the Canadian version has the same style as US - there are two big differences though. First is the front sheet metal is 7" shorter from the centerline of the front wheels to the cowl/windshield/doors due to the difference in wheelbase. Second is that the cutout for the front wheel looks like a Chevy on the Canadian model, while it's rounded as shown in the photo for the US model.
The Canadian 55-57 Pontiac uses a Chevy frame and drivetrain.
The Pontiacs, Canadian or US, share the same glass and window trim model for model as a Chevy. The doors and trunk lid interchange except for the trim holes. The seats and interior trim are the same basic pieces but all the trim is different.
55-57 Canadian Pontiacs are neat cars, I'd love to have one.
What you have pictured there is an american 55 pontiac, the whole floor is different, rear tail pan, rear quarters, front clip, the lower half of the cowl, and the dash, the wheelbase and frame are different.
However if you had a 55 4 door sedan, you could use that roof and doors and the front part of the quarter panels for a 2 door conversion.