Here is where the part of my thread on Henri (my `74 Sov) turns to replacing the original furnace with the one you are talking about, the NT-30SP.
I also mentioned a bunch of other threads at one point, so you should be able to find ways to locate a lot of other threads on the same subject. It is a lot of reading, runs about 50 posts, IIRC.
Lots! of photos.
Take your time, it`s not as obvious as you might think...
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“Courage is being scared to death, but saddling up anyway.”
...John Wayne...........................