Having had a 19T4, I remember (?) that the furnace had metal blanks that were knocked out to attach the outlet flanges. A piece of thin metal could be sized to fit over the opening and screwed or riveted on to block one outlet. ??
As to blocking and overheating, the furnace has a hi-temp cutoff switch that would prevent that from happening. And if you open up another outlet, the air flow should be the same or nearly. If the floor duct is already restricting the flow, obviously the system is still working!!! Had one trailer that "operated" on the cutoff switch when heater running!! Ran that way for years before getting another trailer!!
