The biggest differences between 62 and 64 beds are due to the 62 frame rails being a lot farther apart than the 64. Can you salvage your 64 rear sill so those brackets that sit on the rear frame horns will match your 64 frame? Of course you will have to drill the cross sills to match the 64 frame bolt locations and also drill your wood to match the 64 frame not the 62 frame. The cross sills on your truck have a spacer attached at each bed-to-frame location and you will have to move those to the appropriate locations to match your 64 frame. We have a tech article http://www.mar-k.com/installation_hints.aspx that will help you with the changes needed. Scroll down to the bedside interchangeability article.

Mar-Ktech