Back to the original question:
Hello Scott,
Good for you for wanting to go original in your rebuild of this 'Bubba-d' 1881! The nice thing about doing this in the current day is that you get to choose the dimensions of the chamber and barrel that you want, you don't need to rely on odd old dimensions or averages of existing examples unless you really want to.

While it would be easier and cheaper to rebuild that rifle into a .450 3-1/4" BPE, I have yet to find a record of an original 1881 so built. W-R did build 1881s in both .303 and .375 2-1/2" Flanged Nitro, and the standard 270 grain loading for the .375 2-1/2" was a pretty low pressure affair. If you want a simpler, smokeless solution you might give that one a look. Basically it duplicated the ballistics of either the 450 3-1/4" BPE or the 500/450 No.1 BPE 2-3/4", a 270 grain bullet at around 1900 fps. The .375 2-1/2" rifles I have shot, one H&H and one Lancaster, have been quite accurate.

I've had two original (but not W-R) rifles in 450 No.1 Express, a Rigby double several years ago with Metford rifling and a Field action single shot with Henry rifling that I'm really enjoying now. Both have worked very well with either nitro-for-black loads or full black powder loads. Yes, Bertram brass is expensive; but the current stuff is much much better than those made 20 years ago and how many will you need, after all? I'm using either 300 grain soft cast or 350 grain paper-patch bullets in mine; either one will 'bump up' with black powder to fit whatever groove diameter they're fired in.

450 No.1 Express cartridges with Bertram brass fired in the Field action rifle end up with an outside neck diameter of .490" or a bit over. The neck walls are about .012". The bore and groove diameters? Who knows, it's got that wonderful seven-sided, triangular-land Henry rifling!

The set of Dies I use is from RCBS, in their group "L" dies with a part number of 56618. I don't know if these are still available on special order or not, but a call to RCBS would sort that out. CH-4D also makes dies as you know.

The hardest item to come by for reloading this cartridge is the shell holder. Nothing else I've found quite fits. Again, a call to RCBS or CH-4D should sort that out. I believe the one I have is from CH-4D and I had to do a very minor amount of filing on it to perfectly fit the Bertram cases. Took me about five minutes.

One other thing - in a light rifle, like a single shot will be, the recoil of a full black powder load, even with the light "Express" weight bullets around 300 grains, is quite noticeable. Make sure your stock fits or your cheek bone won't be very happy.