would be interesting to note how old this bird was the saddle feathers seem a bit on the short side which to me is an indicator of a younger bird. Spurs also seem to indicate youth. The shapes change a little as they age and put more meat and fat onto the frame. All in all the shape of the bird is getting there at least in a number of them I have seen. Especially the ones from Kathy's line that Kim is producing.
Color is going to be very hard but shape is getting there.
I was looking at some pics of NH hens the other day and if you bleach out the red in a lot of them they would look a lot like Delawares. The tails and columbian pattern while faint on the NH red is clearly visible on good birds. Thats why I think some trying to "fix" the color pattern are willing to breed Del males back to NH hens with good results. It looks like this outcross might help improve tails and color pattern if your birds seem to be lacking but as Leslie and others are saying your have to be very careful of introducing other elements. Working withing the birds you have would be preferable but sometimes the genetic drift of the line leaves you with little options.
To be perfectly honest Im thinking that recreation and merging of the lines might be a better option.
Im considering getting some PBR and NH from eight acre next year and give it a go.
Saddle length is also connected to tail length. Birds with shorter saddles, also tend to have shorter tails. The Delaware, Rock, New Hampshire etc. have a gene that shortens the tail. They should not have long extravagant tails, and as a result their saddle feathers are shorter. In real life, there is some variability from line to line. In this picture, you see the tail that I am referring to.
You did make a reasonable observation. From this point in his hypothetical development, he would not change substantially. This bird is "finished". He could fill out slightly, but the point of this print, is to demonstrate a finished bird. This is an ideal finished bird. That is the point.
If you want to do any crossing, do it on the side. Breeding is often, "first do no harm". We put to much effort into our birds to lose what we have. So we "play" on the side, and if we are successful, then we introduce it into a family. Not to all yet. Then we make a decision to continue or not. All the while, we continue to move forward with the main flock.
When we are considering something like this, we are attempting to add a trait or two. All of the effort on the side is to maintain what we have while improving that characteristic or two. Crossing strains that are radically different birds is best done on the side.
The getting what you want while not losing what you have is the work of a breeder. It is not for faint of heart, and if you are successful, you have come a long ways.
The Standard requirements for the New Hampshire and Delaware are very similar.