If you haven't gotten an answer by Monday, I'll check when I get into the shop, but.....
You shouldn't need the measurements if you have the monitor with the cross bars mounted on it. The bars are about 3/8" shorter than the inside width of the cabinet. Once they're on the monitor, measure how far apart they are and cut wood to mount to the insides for the bars to ride on. The wood should be about an inch longer than the distance between the outside edges of your bars, although it would make better sense to have it stretch from an inch or so below your control panel cross bar to a close to the back door as you can get with the back door still closing and the marquee, power, and speaker wires still able to clear. That way you'll have room to adjust the positioning. The width should be enough to to cover the mounting holes on your bars. Depth should be whatever you need to ensure solid mounting to your cab. It doesn't have to be exact measurements if this is a project just for you, as most people will never look inside the cab to notice one side is 1/4" longer than the other.
Wait to drill your holes for mounting until you've got the monitor (use a friend) situated in the right place for the bezel. Then use a sharpie to mark the hole positions, remove the monitor. and drill.
Of more importance to the length of your braces is the angle and height to ensure that the monitor is not too high or too low once your bezels are put on. If this is an original cab, then there are probably holes or "outline" impressions showing where they were originally placed, and you can use those as a proper guideline.