Mike, It looks like you have a board kit drilled for a 1/2 ton GMC long bed. 1/2 ton and 3/4 tons had the same bolt spacing front to back but different frame widths, 1/2 ton frames being a lot wider as mentioned in the previous post.
Regarding the angle strip, the original angles extend over the top of the rear sill for reasons nobody seems to know. At the front, they can't extend all the way to the front panel because that would interfere with the bolted flange of the front panel which is about 7/8" wide. That rear hole in the angle strip should line up exactly with the corresponding hole in the rear sill.
Mar-Ktech
