Gerald,
As the tube size increased the cheek, where the trunnion plates were mounted, had to be made taller to allow clearance of the trail stock (center beam of the carriage). So your measurements would not be correct. The distance from the ground of the center line if the trunnions would increase with the size of the barrel. I haven't found the actual construction drawings for these carriages online. But I know you can buy copies of the originals from Steen Cannons in Ashland Kentucky. I'm pretty sure you can order online at www.steencannons.com. I also believe that a quarter of an inch in elevation would make a drastic difference in trajectory for a 19th century field piece. Those guns are suprisingly accurate. Especially the rifled models. Sounds like you have quite a research model in the works.

Keith