Mike,
Based on WilderNest's original sales brochure, you are looking for a topper that also fits:
57.75"-61.75" wide truck beds,
73.5"-76.5" long.
It seems like quite a range, but I have one on my long bed and can attest that the lip where the topper meets the bed is quite wide to accommodate such a range of bed sizes.
Here is the link that I got those figures from.
http://wildernest.armbrusterweb.com/dlf ... st_OEM.pdf
Also here is a link to a forum for the WilderNest. It has sporadic activity with toppers coming up for sale.
http://wildernest.proboards.com/index.cgi?action=login
Lastly, I am in Foxboro, MA if you'd like to have a look at how one fits.
-Steve