Thanks. With the casters I do know what you mean, but I can't see any way of keeping them upright without either adding another 4 casters or a lot of extra complication? It'll never be moved while down, and will spend 90% of its time at full height. The casters are rated to almost twice the load that will be on them, so I'll more than likely see how they go as they are. Replacing them would only involve sticking my jack under that side of the frame, or lifting up that end of the car with the engine crane. If you've got an easy(ish) idea for keeping them level that I haven't thought up I'm all ears though?