Dave, I used these, which I believe is what you are talking about. I used them in the full 5/8 in. thickness and they are a perfect fit for my 3A. The pedals and nice and centered.
Also, I do not have the two forwardmost mounts that you asked about, nor is there holes for them in the tub or the frame. I just have the four along the rear bumper and three on each side of the tub. That's it. Not sure what is "correct", but I've not had any problems at all with just the 10 mounts.