JAnthony,
Without question, heaters require clear, open, unfettered, unhampered air space all around them.
I would suggest you haven't sufficient room to install the heater inside your shed.
Further, it would block access to everthing else in its area.
And you didn't mention whether the heater would be wood-fired, coal, fired, propane, natural gas, solar-powered, or 220/240 electric-powered.
Each of those will have very specific intalllation instructions, and I personally don't know of any that are to be installed indoors. You are dealing with plastics and wood, and coated wiring, which can melt and burn, and I don't believe you can "fire-proof" your shed.
Perhaps someone else has additional info....
Beautiful set-up, great plumbing job, too.
Alan