The stacked bells would likely be the best option, Satamax. I was going to have a very short bench, but that could very easily be converted to height.
Glenn, thanks for the warning about 4" systems. I'll leave that alone. This will be my first build. Maybe I can use leftover firebricks for mass. Heated area will be 420 sq. ft., which is my kitchen and living room (sparsely furnished - I'm somewhat of a minimalist). The house was built in 1953, stick frame, 8' ceilings, oak tongue & groove flooring, crawl space beneath. The climate is erratic here in the Piedmont area of NC. Last winter we had shirt-sleeve weather followed by sudden snow and ice storms a couple of times. This summer has been more like spring so people are saying that means a cold winter. Who knows? Generally, this is a mild climate with an average of 20-25 degree difference between day and night temps, so a massive heater isn't warranted. Moving off grid is motivated by economics and principle.
Edit: No insulation under floorboards, therefore drafty.