Dure hardness is the hardness it sets to.. A low cure hardness is comparative to a bowl of jelly (English kind) while a high hardness is a lump of concrete. This is measured in shore. 75degrees shore is pretty hard, but flexible urethane is exactly what is says, flexible. Normally resins are usually epoxy or polyesther resins which cure harder. This means they're easy to sand etc. Flexible materials don't like being sanded, cut and tend to loose their paint because the paint isn't as flexible. Basically the lower the shore value, the softer and more flexible the resin. You won't have to worry about tensile strength, unless you want to tap bolt thread into it.
Before getting a couple of gallons of the stuff, try out a small sample and compare it to an offcut from a resin kit (the casting mold sprue-thinggy)..