No probs I have:
Ab 47cm
The edge on the kidney that butts against the butt plate is about 58 but my kidney goes past the butt plate by 2 cm on each side
I did find that the top of my kidney plate is 55cm, I found it fitted better this way.
I think you must be heavier boned than me lol, by the weight and height I should have the same need of padding as you.
I could do with a little padding, I didn't want to trim too much off my armor at first, but I did notice my sides popping out a lot so I made some angle shims behind the sides top and bottom that helped to keep the sides in and not wanting to flair out.
I think like most armor makers you just have to make it fit your frame. Think I would try a bit of padding first before removing too much of the sides