Frame width for 5th wheel plate

Hi, I'm going to be installing an above box fifth wheel plate in an '01 Ram 2500 and was hoping to pre-drill the mounting holes before I head over to the guy's house. I found some info stating the ball should be located 48 and 7/16 inches from the rear lip of the box, but I haven't found any info stating how wide the frame mounting holes should be on either side of the ball. My plate is already made and should be plenty wide, so all I need is the dimensions from the center to the frame rails. Thank you very much to anyone who can help me out. I'm hoping to drill through the 1/2 inch steel on the press as opposed to in his yard.

I checked the current holes, which I think are from a 1/2 ton chevy we used to have, they are 35.5 inches centerline to centerline of the holes.

Needed to change the hole centers to 35 inches. Just slotted the original holes a bit. Some spacers between the bed and the frame and it's all good.