1988 Chevy K3500, 350/TH400

Had a winch bumper fabricated. All the guy wanted was my year, make and model. Bumper shows up to be mounted between the front frame rails 28.25" width. My problem is my front rails are exactly 27" apart (inside edge to inside edge). Can't find out what the factory specification is. Can someone give me their measurements or point me to the specifications on the internet?

Thanks