Since cannons barrels are tapered the cheeks have to have draft so the breech will fit between them and the rim bases will seat against the cheeks
Taking the top step of will make an immense difference.
The trucks-wheels need to be a smaller diameter. Often the rear trucks are smaller than the front.
Take a look at the pictures of the carriage on the HMS Victory to see what I am talking about.
http://www.houseoftheorangemonkey.co.uk/monkey/trips/trip1461615.htmHere is a carriage on the U.S.S. Cairo
http://www.nps.gov/vick/forteachers/upload/Great%20Guns.pdfHere is my 1841 42 PDR model before refurbishment made from drawings from the Original gun .
Draft
Here is the original gun and you can see why you have to have draft.
If the cheeks were parallel the trunnion probably wouldn't t reach the cheeks.
This sort details not only make your carriage look a lot less amateurish, but will give you a stronger more stable carriage.