Well, I suppose it is a good thing that someone wants to cut up a Sunset "J". They had a lot of detail, but they were not in the category of "great" brass. Still, they can be sold for around $600, and the very nice Williams with baked-on paint can in turn be bought for around $300.
If you do it yourself, you will definitely need a narrower frame. Depending on how Sunset detailed the frame, that could mean a new frame. If you plan on using it on 3-rail typical curves, say, O54, you will need to remove the tail beam and find a smaller set of cylinders. Often the front frame will need lengthening. A longer drawbar is trivial, as are the tender trucks and dremeling the rear of the tender to accept the giant coupler. Your locomotive will then be worth about $100 if you ever decide to sell it.
You will learn a lot, though, and that is priceless. Let us know how it goes. I shall keep my Williams 2-rail "J". Very happy with it.
