My blades are about 24 inches apart, I can check in the morning and post.
About the straight set blades, I felt for most of my work which is planing landscapes that the straight set blades were the best deal. I don't wan't the dirt to drift to one side or the other. I also have the blades set to 0 with the bottom wear strips. Since the plane is heavy it will still cut aggresively with a slight shortening of the top link. I can go a short distance and easily have dirt piled up under the tractor behind the rear wheels. That isn't the purpose of the landplane for my use though, if the ground is flat and smooth I want to move very little dirt. My thought is to cut off the mounds and fill in the depressions. I also have wider skids to reduce the gouging. Overall I am very satisfied with it for land planing in small to larger lawn areas.
For primarily road and driveway work angled blades would be preferred since you can tilt up the rear slightly and build a crown. I use a rearblade with topntilt to build a crown and recover material from the edges. I have had good success using the two implements together for roadwork. I do have the topntilt which allows me to raise one side of the plane slightly to help with sloping when needed. For roadwork I can see having the blades set lower to better mix the gravel and fines.
About the washboard areas, so far these seem to be dealt with easily and haven't noticed any issues or experienced any problems removing them.
About the DIY side of this project, if you think having adjustable depth is a real concern then an easy work around to address this problem would be to build the plane with the blades 1 inch below the skids. You could have pin on wear strips that would protrude upwards thru the flanges and use a hairpin clip to hold them in place. Have a set of thin wear strips 3/16 to 1/4 inch which would allow a net 3/4" setting below the skids and a 3/4 inch spacer wood strip to sandwich between when wanting to finish plane at the zero setting.
If you want to test this using flatbar in lieu of grader blades I would strongly recommend you drill the holes needed for the grader blades prior to assembly. It would be very difficult to drill these holes afterwards. Frankly I think all of us that have these implements will tell you to simply go for the grader blades to start with.
Last thing, Brian and I have talked about using these in reverse, both of us do. I have contemplated putting a hitch on the back end and then dragging it. Easy to drive forwards and safer for the hitch components too. Debating whether to do this or add the flip over tailgate with stops.