My feeling would be to go with the "current"standard" .375, if for no other reason, to lessen the headaches involved in larger barrel dimensions, such as mould selection, or jacketed bullet choices, subsequent chamber choices, etc. To be historically correct, or "incorrect" as the case may be, pick and choose, and go merrily forth. A long time ago, I had William Large make a barrel for me, and was glad he chose the .375 groove diameter, I have enough .380 to .382 barrels, but this choice is yours. Whatever gives you satisfaction, if nothing more than the fact that you did the work yourself.

So, I've ordered a .368" bore reamer from Pacific Tool and Gauge, started on the rifling head and plan on cutting grooves to .375".
Planning on shooting BPCR with it, from earlier comments in this and other threads here, I've also ordered a Lyman 335 grain bullet mold. Of course, everything is on back order.   Was thinking 12 or 14 twist because of the heavier bullet but I haven't made the spur gear yet to produce the twist so I'd certainly entertain suggestions from anyone more experienced than me.
Almost ordered a modified Manson Mcpherson chamber reamer but stopped because I thought I should maybe wait until I have the bullet mold and bullets "in hand".

"Cue the jaws music."
Indeed - for my 45-70 I have the Lyman 535 gr. Postell bullet and with 67 gr. of black behind it - it can be kind of punishing.