What's in there now? An NT-22, NT-30?
Whatever it is, why go for anything different? However, I think you will have to build a small platform to raise up the carcass of the furnace, since it seems that the new models are a few inches lower than the old ones. Building the platform means you can stick with the same two holes in your AS' body, rather than cut new ones.
If you want some reading about what I needed to do to Henri, my '74 Sov, to get a new Suburban NT-30 installed have a read through that part of my thread on Henri starting here:
“Courage is being scared to death, but saddling up anyway.”