What ever you use for spacers you want to make sure it's a smooth and close fit with the bolt diameter so it slides right over the threads and the hole you drill is a match to the outer diameter. I used the wedge plates clamped in place as a guide for drilling the holes, and then made the holes larger to accommodate the spacers, but it still may not be perfect for all 4 holes to line up with the backing plate. I had to drill the holes in the backing plate to be a bit wider, but well within the tolerance of the washers to give a little wiggle room for all 4 bolts to make it through.
As far as placement, I went a little forward of the back corners. You want to take in account where and how your canvases are installed. I made sure that the handle didn't get in the way of any zippers or where the openings of the hanging slots for the canvases are located, as well as rolling up canvases. I think but can not be sure, if you look at where Grady is now welding reinforcements to the hard top frames it's a bit forward; however, I have a walk around so I have an easy reach in that location. Your Express might be a different scenario on what placement on the hard top will provide the easiest use, so I would take that into consideration.
I have seen outriggers installed at the back of the hardtop inside the width of the frame. I've also seen them placed so the handles would be inside the canvases. Those placements would determine if you need the wedges or not, but also might be convenient with your express.
Here's a link to the wedge plates I used (should you go that route):
In case your T-top has a slight curve, keep the appearance of your outrigger bases clean by mounting them perfectly straight with our plastic wedge plates.
tacomarine.com
Another thing you can do, is walk around some other marina's or GW dealers and see where and how the hard top outriggers are mounted on similar Grady's to yours.