You might also get away without the wheels. I built my base frame out of heavy wall steel box tube and it just set on the garage floor while I did the initial inspection, test run and balance. When I needed to move it, I could do one of two things. I could attach my lift jig to the head and lift engine and base frame with an engine hoist(the frame is a pretty small fraction of the 700+ pounds of 6/1 engine). I could also lift the frame off the floor with a pry bar and slide a wood block underneath. Once I got it up high enough, I would slip pipe underneath and it was quite easy to move this way egyptian style. Once up on rollers, I could push the 1000# engine/frame/generator across the garage floor with the tip of my finger.
For my finished installation, I bolted that same frame to a 1300# block of concrete just outside the door to my generator shed(put the engine/frame inplace with my loader on my tractor). Then I added the generator head. Using a longer prybar and fulcrum, I lifted the now over 1 ton assembly vertically onto blocks, then onto 2" steel pipe sections and three 2X4 rails as it was on grass in front of the shed. I then could, using the prybar against the ground, quite easilly roll the whole thing thru the 30" door(block had 1/2" clearance with part of the door frame removed). Don't underestmate going old school on it. I did the above operation all by myself, I could set on the pry bar to lift the base block and push the blocks and rollers into place with a smaller pole. Once it was in the desired position, I reversed the process onto wood blocks then down onto a rubber mat. I think I would have had a real hard time moving this thing into position on caster wheels, because ALL the weight will be concentrated on just a few square inches where the wheel contacts the surface. But I could have moved it from garage to shed with pipe rollers...
I think Archemedes said it best, "Give me a big enough lever, and I wll move the world" Here is the generator in place in the shed. My back is in the corner next to the door holding the camera above my head to get the camera far enough away to see the whole engine. It is not a very large room...