Interesting that I found this. I remember you mentioned this in another thread so was waiting for a post about it. I did the same thing this past weekend with my 240 gallon tank although it only weighed 400 or so pounds. I saved the shipping crate and pallet it rested on so knew I had the proper size. Ordered 400lb castors, repurposed the 2x4's and used some clamps to sync the legs together so I could slide it off the steel stand and onto the temporary one I just made. It doesn't look as strong as the one you built but then again you are holding 1000 lbs
The good news is that once you are done you will have a pretty slick roll away work bench. That is what I'm doing with mine. I'm going to counter sink my mitre saw in it or possibly another such tool and maybe my shop vac.
Great job. You are a pretty darn handy and creative person. Really.