I thought you might be interested in this link too.
sorry about that- links not working properly, sorry. Only have a source for quarter panel. Actually I'll post that link too incase you were interested in that as well. They have quite a bit of quarter panels for classic cars, but no fenders. Quarter panels
You are right, the extra length of the Bonneville is in the rear. Front fenders are the same for Canadian & US production. There might be some difference in the internal bracing as the inner fenders might be different, but external sheet metal is the same. I have a Canadian Pontiac, but I used the US body manual as a guide in dismantling it.
the fenders won't fit. While the outer skin may be the same, the US Pontiac inner fenders and radiator support assembly is entirely different. While the 1970 B-body US and Canadian cars look the same from the outside, once the hood is opened the differences are obvious. To my knowledge, the only front items interchangeable are the bumper, valance, headlight panels and likely the hood. For the right price, it may be worth experimenting with the US fenders on a Canadian car.
