Derek is right in only part of his reply.
The original CMP Field Artillery Tractor (FAT) was a #11 cab and was fitted with run flat tyres. As production of vehicles increased the production of run flat tyres fell behind so ordinary tubed tyres were fitted along with a rack on the sloping rear portion to carry a spare. Only the later #11 cabs had vents in the front facing panels.
Next came the #12 cab which was basicly the same apart from having a genuine bonnet (hood) and all 12 cabs had vents in the front facing panels leading to the cabin of the vehicle. Ford #12's had square louvred grills over these vents while the Chev had a round meshed hole. The Ford version of this is represented by the Tamiya Quad kit.
Then came the #13 cab with reversed slope windscreen. These are the most common of these models and are represented by the Italeri Chev gun tractor kit.
At the same time you also had the Australian Ford #8 and #9 body styles on #13 cab style chassis.
To build a Canadian version OOB you can use either the Tamiya Quad kit which is more 1/32 scale then 1/35 and has a lot of inaccuracies or hunt around for the Italeri Chevrolet Gun Tractor kit (#240).
The Italeri kit was the most common vehicle type used and also the most accurate of the two availiable kits and, while being OOP is still to be found around the place.
Eduard I believe had PE for the Italeri kit at one stage but I am not sure if any was/is availiable for the Tamiya kit.
cheers
Cliff