How about fabricating the stand with adjustable legs ? Drilling rows of matching holes in the bottoms of the legs and in short pieces of stock used as bottom extensions will allow the height to be adjusted as needed. This was commonly done on some power tool bases or stands. Using bolting in drilled holes rather than slotted holes insures there is nothing to slip in the connections at the bottom of each leg. It also insures that the legs are all the same length.
This idea works whether the table is to be stationary or on casters. I've seen some very heavy granite surface plates set on stands made of steel angle, with casters on the bottom of the legs. This allowed the plate to be moved around as needed so access to all 4 sides was possible. It also allowed the plate on its stand to be pushed out of the work area when the floor space was needed.
On that same note, adding some levelling screws to the bottoms of each leg with swivel pads (stock tooling components ) would be the ideal if the table were to be left in one location. The swivel pads or "feet" can be ordered with threaded studs for shanks. This will give maybe 1/2" of adjustment for level. For fine inspection work on a large surface plate, having it levelled is a good idea.
Either way, the adjustable length legs could be combined with either casters or swivel pads. Making the table with adjustable legs having bolted connections is a lot neater and more solid than finding out the table is too low and having to put blocks under the legs. If the horizontal crossmembers are located at a convenient height between the legs, a bottle jack could be used to raise or lower one side at a time for height adjustments on the bolted legs.
