Actual Dimension of SWB Truck Bed Needed -- Help!

[1986swb]

Hey folks. My 1986 GMC Sierra Classic SWB (6.5' bed) is in the shop so I need your assistance in obtaining the actual dimensions of the truck bed.

I need the top rail bed measurements:

1. Front to back edge -- top rail to tailgate
2. Side to side -- top rail to top rail

I hope this makes sense.

A buddy of mine is selling his hard-top tonneau cover but it is from a later model year Chevy truck. I need to see if it fits.

Thanks everyone for your assistance and support.