That's Cool man ! My Old Man put one of them in his 77 Avco Bus,,, That thing would FLY for a 38 foot camper. You could also put it in low,Low and top end was walking speed. Your gona like that thing man.
Are you going to replace those later on befor driving? looks like they will get grushed over time with the weight of the truck. I have used 2x4 and 2x3 boxed tubing with the openings on the top and bottom before with out a problems. You can also weld plates over the openings to add the strength needed. hate to see you go around a corner and have them colapse on you.