Alright - I'm making a little progress now. I bought a deuce and a half, so I have 2 projects going on at the same time. I got my red oak blocks cut and I laid the measurements off according to the drawings above. I saw where the measurements should be adjusted to my mount legs, but I wanted to double check a couple of things before I started drilling.
First, according to the measurements, the blocks are 14" long. The mount is only about 12". Do I cut the blocks to the length of the mount? The pictures that I have found don't show any wood sticking out past the mount, so I'm guessing this is what I need to do.
Second, the holes in the drawing are nowhere close to my mount holes. Mine are less than an inch apart.

So, do I use all 4 of them or only the 2 closest to the middle?
The other measurements and wheelwell channels are close enough to use as is.
Thanks,