I've done what you want at least a dozen times for my old model Rugers and some of my friend's.
First the grip frame: as Contender posted above, there's no old model XR3 size steel grip frame. But there's plenty of New Model XR3 size steel grip frames used on the New Vaquero and New Model Flattop Blackhawks.
The early NM XR3 frames until 2012 have the internal lock which doesn't affect installing it on an old model.
If you don't want the lock, get one that looks like your grip frame internally. They can be found in this forum's classifieds or post a want ad there, or ebay and gunbroker.
They usually fit to your mainframe very closely but not perfect, except for the grip frame "ears" on both sides of hammer: they'll need shortening of about 1/16". You can do the fitting of the grip frame to your gun and touch up with cold blue or have it reblued. Another option is a stainless grip frame: you can fit it and and then polish it. Some like the look.
You can buy a new XR3 from Ruger but it will come in the white, need a lot of fitting because they're oversize, and then have it blued.
THE ONLY ADAPTATION NEEDED FOR A NEW MODEL GRIP FRAME TO AN OLD MODEL IS THE TRIGGER AND RETURN SPRING as contender mentioned.
Another very much simpler option:
You can use a fired .22 LR case to hold the OM return spring and plunger in place. Use the NM trigger return spring retaining/pivot pin closest to the trigger shown below to hold your OM trigger return spring/plunger and .22 case in place. File a small notch in the .22 case head to nest on the retaining pin. I've done several like this and they've never failed.
The NM grip frame will have a wider trigger slot than the OM. Use an old model wide Super Blackhawk trigger to fill the slot if you want.
The steel ejector housing is easy to find at the places above or order new from Ruger. You want one with the small screw hole same size as yours. All the NM housings use the small hole.
Below is a grip frame made for the lock and another photo lower down bottom showing a Bearcat spring seat that will replace the lock to eliminate it, and at the bottom a home made bypass
Hope all this helps,
