Thank you all for the great input.
With a special thanks to Wayne59, who allowed me to visit him Sat. to examine his very nice Deeley and Edge patent target rifle. I picked up some some good points on WR styling from examining his rifle.

The extractor on my rifle was most likely made by the "gunsmith" who did the rest of the work. The fit and finish on it is not on the same level as the rest of the internal parts. But it does function well with the 45/70.

Not having to make a new extractor was part of my thinking when I started considering the .450 BPE or .450/400 BPE chamberings since the rim dia. is just slightly larger than the 45/70. The other considerations were that brass, dies and chamber reamers for these calibers are easily available.
The easiest thing would be to just stick with 45/70, but this rifle deserves to be in an english caliber.

50Calshtr makes a sound point about someone in the future, once the gun has passed from my hands, slipping a NE round into the run.

Seeing in Winfer's book, which I have, that several guns with serial numbers close to mine were in .577/500 No 2 Exp. led me to believe that was the source of the "Express Rifle Patent" marking. With the input I have gotten here I am believing the .500/450 No. 1 Exp. is probably correct.

I will have to search the forum and see what is involved in making and loading this round. Anybody have a chamber reamer laying around they want to sell or lend out?

I have directly contacted a couple of forum members who in the past have posted photo't of their 1881 sporting rifles. One member no longer has his rifle and another has a medical issue and will not be able to assist me for several weeks.

So if anyone has an 1881, or 1897 sporting rifle with a round barrel and would be willing to share with me the details of the barrels exterior contour I would be most grateful.

Thanks again for all of the great input. I will keep you posted on progress.