I had a couple body dollies welded up when I was doing early Corvette restos. They were 2" angle iron, with legs down to the lockable wheels. I made them about 16" tall, so I could get underneath to work or paint.

The one thing I did, that you could incorporate into your design is, I used vertical pieces of 1/2 inch threaded rod to support the body. That way you can drill new holes, and move those rods to fit whatever you are working on. I still use them today.... 24 years later.