Hi Doc - Building the coach was quite a job, but fun. All of the outside dimensions are from the original plans, except the wheelbase (see below)
I got the tires from Northern Tool and Equipment,
www.northerntool.com 155121-1505 for the 20 inch and 155123-1505 for the 16 inch.
I cut the lumber for the frame on my table saw from fir 2x6s. I just found the cleanest and straightest at Lowes and trimmed them down to to 1 1/4 x 1 1/4 inches (actual) so there are no knots. The coach box is 3/16 birch plywood glued and stapled (with an air stapler) to a 1x1 inch frame made from frame leftovers. To keep the center of gravity low with the lightened coach, I mounted the body lower, but this caused the wheels to hit on a tight turn and the body to climb up on the rubber tires, tipping it over. I didn't discover this until the box was all finished, so I moved the front pivot point forward until the wheels cleared. If I was doing it over, I would trim the body to clear the tires or mount it highter to keep the wheelbase short for better maneuvering. The whole thing is screwed together, so I can take it apart and change things as time goes by.
The sides of the gun rack are also 3/16 plywood, the top and bottom pieces are made from 1x6s. The roof is a 3/8 x 1 1/4 frame covered in 3/16 plywood. I then covered this with thin polyester batting and tan vinyl. It gets too hot here in New Mexico for a black roof.
The axles are 1/2" all-thread mounted in a groove dadoed into another piece of guess what? Clear fir. They go all the way across. I put the wheels on with pins to make them and the axles removeable. This thing is wide! It will not fit through a normal doorway assembled. I'm going to have to a little shed outside to store it in.
The wheels are made for bicyles and have very nice bearings, I may have to put a real brake on it, it rolls so easily, I left off the fake one to save weight. Also, the hubs on these wheels are not as wide as the originals, but I kept the same outside dimensions (width) for stability.
I'm working on a web site to post more photos on, but it will take a while. I've got a hot rod to finish now that the stagecoach is done.
Good Luck,
PP