I finally installed the Suburban NT-34SP in our '71 Safari this past weekend.
I removed the outer cover of the old furnace and removed the rails in the plenum. I ended up making a wooden frame, screwing it to the front of the plenum then covering it with aluminum tape. I fashioned an aluminum 90 degree angle to block the bottom front area of the plenum.
Unfortunately, the furnace exhaust wouldn't line up with the exhaust holes in the outer skin. It needed to move to the right about an inch, so I bent (and cut) an inch of wall paneling back to allow the furnace to line up with the exhaust holes.
Gas and electric are connected and the new thermostat installed. The unit cycles on and off correctly and is very quiet. Warm air was a welcome end to this project. Hot Dawg!
I used the bottom left duct open to route air under the range. That should help heat the mid-section of the coach in the winter. As others have noted in various threads the output in the rear of the trailer isn't very powerful, but it is in the front. The air flow under the sink should help balance out the temp in the trailer.
I want to thank the many folks who have posted info on replacing their furnaces in various threads on the forum. It wouldn't have been anywhere near as easy to do the work if I didn't have access to the information you all posted.
I have the old NT-22A if anyone is interested in it. Pay for shipping and it's yours. Local pick up is also welcome.