Joe,
Since you already have a cart from HF you only need to make a small modification to it to have it functional. Here a re a couple of pics of my side shelf that is on my bigger roll around cabinet. I used a couple of shelf supports for closets that are fairly heavy duty and added a double layer of plywood for the shelf. Mine has a 4-1/2" vise mounted so I can use for Free wheel removal and holding stuff steady. The side shelf should hold your truing stand nicely and won't interfere with the operation of the top storage space. You probably have more than enough weight in the cabinet to keep it stable. The foot print of the shelf should not be too big of a problem for your overall shop space. HTH, MH Gimme a call if you need help on this one 812-336-3283
Home made side shelf on one of my cabinets. Note the vise mounted and the heavy duty shelf brackets. This is on a MAC three drawer cabinet with a slide open top for additional storage.