Bob,
The things you need to consider on the mule is weight and height. The taller it is, and the more weight you get on top the more likely it is to turn over, or at least be unstable.
Muffler pipe is good to use because it is stong and lightweight. Also some light wall square tubing is good.
I built my first chair in the early seventies, and have re-designed it several times. The one you build will be changed up after you use it a while and see things that will make it better. It's been an evolution of sorts, needing to be able to hunt with people or by myself, ideas seem to come every year, and I cut and weld on mine several times a year.
You might even consider some swing down jacks on the corners to help stabalize the added weight and leverage of the chair. Somethins as simple as portable jack stands, to electric leveling jacks like on a motorhome. You can do as much or as little as you wish. Just have fun and be safe.
Randy