Yes I read an article about a forge built based on that design a while ago . The article mentioned that the intake temperature could excede the ignition point of propane so it was nessasary to have more control of the propane injection . It briefly mentioned the problem had been dealt with be using injectors from a diesel engine as I recall.
What do you think of putting the exhaust behind a concave shaped movable back wall made from a castable refractory coated with an IR coating with a gap around the circumference for the gases to pass through or would a convex shape be better . My thought was to mount the wall to a threaded rod of some material that I have thats similar to iconel.