The last few I did I charged $450, and they were happy to pay that.
Good SWB frames are a rare breed in these parts, so when you do find one in good shape the owner wants a mint for them, so shortening the more available LWB frame is usually more cost effective.
I'm not going to try to talk you out of doing it your way, because it
will work. But, me personally, if I were to see it at a show, I'm that irritating guy that would be shaking his head going "why did he do it that way??"
It would be obvious to an enthusiast that knew what things should look like under there that it was done in a weird way. But building these things are done to the builder's satisfaction, not the onlookers.
Don't take this as a slam against you, its not. I'm just trying to give you my reasoning why I would rather shorten the frame in the middle -vs- moving the x-member up. It should be all about what you want, not us.