I run weld in plates, and prefer them because they absolutely don't move. The same hole is cut for bolt in and weld in, or very close to it.
One other thing that I have on the ones I make is that the slider is steel. The advantage of a steel slider is that the bolts are Tig'd in. With the aluminum, the bolt can back out as you try to loosen/tighten the nuts to change the alignment. This can be a royal PITA! You will see folks have a slot cut in the end of the bolts on the aluminum sliders. Lots of fun!!!

Mike