Out of curiosity, how much collectors value does a single six have?
I can see not wanting to mod a 18something colt SAA, but the single six doesn't seem like a collectors piece so much as a shooter anyway. Since they're neither rare nor historic, and are still being made, I'd think there would be a limited number of people who would be willing to pay more for an old gun then they would for a new one (I certainly wouldn't).
But, I've never been in the market for a used single six, so I could be wrong.
Also, if you're planning on giving it to your rather than selling it, what do you care what it's worth to a collector?
Personally I'd get the mod done. Not because it's a boy-killing death machine without the mod, but because the mod does make it a tiny bit safer. And, I do lots of annoying crap because it makes things a tiny bit safer for my boy.
But, it honestly doesn't seem like that big of a deal either way.