You are right about it takes one to make one.
Another observation I have made is the smaller the rails are, the less forgiving alignment is. The first ones I used were THK size 12. They are on the Z of my router. They were a bear. After the first try with the aluminum plate on the trucks, I made a template out of lexan and mounted that. The second template was perfect, so I flipped the original plate over, drilled new holes and they turned out perfect as well. Luckily, I didn't need any shims. If the axis will work from gravity alone, that is close enough.
The next rails I mounted worked much better and worked the irst time. They were size 27's and 25's. I did change the design on the mill to accomodate the type of mounting the trucks had though. Apparently I could not build them with the axes in the correct position starting at the bottom up on my new mill. I have to remount them. Not enough forethought on my part. It did give me some practice though.
I have seen some nice aluminum plate and some that was really bad. I have found that using a ROS will at least help to level the plates and knock the burrs off. Bars are even worse than plates and 80/20 stuff is worse than all of them. Once my router was done, I have used it to surface everything for my mill. You may just have to get it close and move on until it is up and running. Then make new parts with the machine running to make it more accurate. You may find that it will work great with shims or with a little larger holes in the trucks plate on one side. I mill aluminum with my router quite often. It works great.