Intriguing idea. The Williams was pretty good at its price point. Not sure I would set my standards as high as Key - the lost wax parts alone will cause you some budget problems. There are a couple of small areas where it will take extensive work for not much return - the tender oil tank, for instance, is a separate piece, held in with what I would call door dogs.
I concur with the frame extensions - that is an easy change, and really makes the model look good. I have a 1941 Lobaugh that still needs that treatment.
And on the decals - I can steer you to my decal guy, who can supply enough to do quite a few SP locomotives for ten bucks plus shipping. Maybe by now Protocraft has them, too.
