Bill, the sander is held in place by three strips of wood or cleats glued and tacked/screwed to the upper surface of the carcase in the form of an "H," with the horizontal bar in the H sized to fit the miter slot in the sander's table. The two vertical bars of the H fit just outside the sander's table and the three together keep the sander from moving. Originally I had used just a single strip of wood, but someone suggested the additional strip to fit the miter slot and that is indeed better.
I subsequently added a third piece of 2x4 to the mount for the jointer so that the floating table of the 510/520 system could be stored front-to-back under the infeed table of the jointer. But, if you have the dust collection fitting on the bottom of the jointer, it doesn't fit unless you remove the fitting. So, instead of adding a third layer of 2x4, I'd make two small blocks with 1 1/4" holes so there is a gap in the center to accommodate the dust collection fitting. I haven't done this, so it may require some addition tweaking to make the jointer fit with the collector in place.
I also added a piece of 2x4 with holes to the bottom level for holding the lathe tail stock and tool rest. The tail stock uses one of the holes, the tool rest the other.
The bandsaw is heavier than the strip sander so I though that higher was better for the heavier piece. Moreover, the whole thing was designed for the Special Purpose Tools I have. There are other examples of the cart made by folks with different tools, so you need to design it according to what you have or expect to have down the road.
One possible addition would be to store lathe chisels in the top drawer and add a couple of cleats across the bottom of the drawer so that it could be removed and placed on the Shopsmith's bench tubes or way tubes when turning. Center those cleats and space them so they fit just outside the bench/way tubes and they'll keep the drawer from falling off the Shopsmith while turning. This may require some adjustments in size of the drawer boxes so that the cleats don't interfere.
I had the 3/4 drawer slides on hand, so I used them. If you're buying them, perhaps full extension slides would be better, or you could dado the carcase or the sides of the drawer boxes and add some rails to avoid having to buy commercial slides.
You also could think about some caddies for holding saw blades, or design some compartments to house the blades so they're not banging into one another. Or resize the drawers to hold what you have.
I made a three-drawer cart that fits under the Shopsmith itself, so between the two I've got plenty of storage for Shopsmith items. See
https://forum.shopsmith.com/viewtopic.php?p=57962&postcount=9
