100826 is a rear pan with license tag box. The pan won't "bolt right up" because you have to notch the rear sill first so the tag box will fit. Then it can be screwed to the rear sill with no further modifications to the bed or pan other than pre-drilling holes for the little screws.
See the following link on our website for instructions:
http://www.mar-k.com/Catalog/PDF/GM%2034-87%20Step%20RP.pdf

Hope this helps.