Ron,
If I remember correctly, and maybe Paul can take a look out the back door and confirm or correct me, the bottom of the #3's firebox is flat at the back and has a mild slope at the front, but the #4's firebox has a steeper slope, starting at the back. To fit the #4's boiler to the #3 would involve building a new frame and a new cylinder saddle, essentially making it a replica #4.
One story I heard was that the #3 was used for bucking snow. All the frame breaks with huge blobs of weld repairing them certainly give credence to this. Houghton gets over 200 inches of snow every year!
Given all the things that are wrong with the loco, you would be better off just building a totally new loco around the #4's boiler and reassembling the parts of the #3 for display. Now, if some Colorado crazies wanted to finance such a reproduction, they just might be able to get 464 in trade. Upon completion, of course, and probably over one or two dead bodies! ;-)
Michael Allen