Re: Building a mono-shock frame to mount any size tire or engine.
I say a custom frame to accept your tire choice and a seat with a good mono shock built in would work fine.
I am not sure how much tubing you purchased but for what you paid I could get you 20 ft of 3/4in, 20 ft of 1in and 20 ft of 1 1/2in, thats enough to build three frames. All this round tubing in .090.
Plus that .090 is easy to weld vs that bicycle tubing...