If your problem is not with the backstrap, as you said, then try using a grip adapter. This will position your "upper" fingers further forward, and reduce the stress on your pinky (which is what I understand from your post you are complaining about). Tyler T-grips are still made, and pachmayer and mershon made units that are long out of production but available via gun broker or ebay. This would be a better first step than hacking on your grip frame.
I agree with Old Fluff - converting a square butt S&W to round butt is easy. But I also agree with SaxonPig and RCmodel, don't go try to re-invent the wheel - selling it and buying what you want is usually best, unless you know you are holding in your hand the perfect revolver and a round butt is all it takes to make it that way.
I have various frame sizes and butt configurations, as also do SaxonPig, RCmodel, and OldFluff. Try some different grips before you hack on your frame! It sounds like you have oversize targets on the gun and are working them down? Did you just try putting some magna grips on it and trying it that way?