Here is a copy of the No.1 Express from the 1884 Kynoch catalog.

Here is one of my GG loads next to an original cartridge.
My load happens to be with Bertram brass but I also have used original Kynoch cases as well as brass made from 500NE and 470 brass. All work fine if headspace is off the shoulder. Once they are fired, the neck is long enough to maintain concentricity with the chamber - that is if you don't resize your fired brass.

The original has a neck of .483"
A patched bullet of .454"
The base is .574"

The Kynoch measurements may actually be for chambers - not sure.

One thing you may want to consider is finding a good condition Martini Henry barrel to turn for your rifle if they are big enough to get the right dimensions. That way you would have he correct Henry rifling.
I will note that Westley Richards used Henry, Whitworth and Metford rifling In their rifles in the late 19th Century.