Okay here is the excel spreadsheet I was talking about saved as a word document so the sheet is not active.
To come up with the multiplier, I divided the major diameter of the cannon I am copying by the diameter of the material I am using. In my example the material I am using is 2.5 diameter. In your case the material diameter is the major diameter of you cannon barrel.
When I divided my material diameter into the major diameter of the cannon from it's drawing. That number is my multiplier.
Use the multiplier to to create an excel formula. Here is the formula I used in this example.
=PRODUCT(C7/5.52)
What this formula says is show the product of cell C7 divided by 5.52. 5.52 is of course the multiple for this example
In my example I put the numbers in for the barrel in the first column. In the second column I copy and pasted special the formula and pulled the formula down in the column which did the conversion.
You can do the same thing with the carriage dimensions.
In your drawing you can remove the rear step and move the hind axle forward. You can also shorten the front of the carriage ahead of the trunnion. The flat front of the trunnion should be shorter than the flat rear of the trunnion.
The cheeks should also taper. Distance between the front of the cheeks should be slightly less than the distance at the rear. Wider at the back narrower at the front
I can't recommend Damion Siekonic's great book
"Building 18th Century Naval Artillery" enough. He also describes a method using projection to scale carriages. It's a great book.