You are going to have to make that decision based on what you did to it. We have no idea what engine, transmission, rear end, springs or brakes you put on it. We have no idea how you stretched the frame, or the quality of the work and engineering. Trailers have brakes so you need to install a quality brake controller and a properly engineered hitch receiver. You could use guidelines from modern vehicles that have similar components to what you put in it. As a WAG, you are probably going to be OK up to around 8K lbs on a trailer, maybe a little more on a fifth-wheel. So many variables... Sounds way cool, though! Is it a first series or second series - model changed mid-1955.
Brian