Body dolly design plans

[BBC71Nova]

I've been tossing around the idea of building a body dolly for some time now. I originally planned on a square frame design but after thinking it through it seemed like it would be more difficult to get the body mounted to it with jackstands in the way. It would also restrict the ability to work under the car. I think I've decided on an I frame type design since this is not only easier to build but would also allow almost full access to the underside via a creeper. That will come in handy for floorpan work.

I'm planning on the basic design as I've drawn it below unless you guys point out some serious flaws. I want it to be adjustable in all directions. This design will allow me to swap in different vertical pieces with different heights/mounts as needed. I'm planning on using 1/8" wall mild steel box tubing for everything. I found some 8" locking, cushion tire casters with 450lb rating which I think will work. I'm no engineer so...