Tony;
How bad is the corrosion on the back side of the lock plate? If it is no worse than what we see on the front side, I reckon that I would have a go at welding and polishing and engraving afterwards.
Whilst re-color case hardening is a task that you are far along in learning how to do, I suggest another route due to the very fine engraving you still have showing on both Grants. That would be to re-engrave the welded area after a proper polish and see if both guns look brilliant just the way they are without re-color. To my mind the Grants (which are sterling guns) do not look as lovely color case hardened as they do with the actions hardened and in the bright (somewhat French Gray). At least the 12 bores are that way to me. The small bore Grants are a bit different visually and the color case hardening looks OK.
By-the-way do either of the Grants have the initials "J.R." stamped in the back of the action between the strikers?
Good luck in whatever you choose and take care of yourself
Best Regards;
