I'm glad to see Hondo's post. It's good info.
I too was confused about your intentions when I read your posts. It's a simple mis-understanding because of terminology.
There is the "main frame" which is the serial numbered part and houses the cylinder. Then, there is the "grip frame" which attaches to the main frame.
Since you are trying to fit a New Model, grip frame,, to an existing Old Army,, that uses the Old Model trigger return spring assy, it's not a complicated or hard conversion.
Hondo mentions using a spent .22 Rf case to be used as a trigger return spring "socket" that's similar to the Old Army/Old Model ones.
Here's another direction to do the same thing.
You can make an adapter to fit in the New Model trigger spring channel that works very well. I've made several over the years. If you want to go that route, look at the pictures of one here;
Now,, not shown (easily,) is the hole drilled into the square end for the return spring to fit into. The angled part is for the contour angle of the hammer strut/spring assy. The small hole is to be used to hold the adapter in place,, via the hole in the grip frame where the pin for the legs of the NM trigger spring would hook. The odd opening in the adapter in the top is just where the drill bit got a little off as it was being drilled.