Congratulations on a great rifle. Hearkens to a classic African time, with a Farquharson style action, and the classic African rimed Cartridge. I owned the same for several years. I immediately put a Red Decelerator on mine, looked really nice with the figured wood.
With Hornady factory 400 grain and my matching reloads, it kicked me like a mule! I am very familiar shooting big bores, .458 Win Mag Whitworth, .416 Remington in both a Model 70, and another Ruger #1, and others. I think it was that the stock must not have fit me well.
I tried 300 grain Hornady .411 SP (for the .405 Winchester) and it was much more manageable, about like a 375 H&H. I got this idea from an article in Rifle magazine by John Barsness. I used the 75% rule, same powder charge as the 400 grain load ( I used a book load of R-15) and a bullet 75% of the weight, hence 300 grain.
I ended selling the rifle (now I regret it!) as my plan was a 2 rifle African battery, pairing it with my 450/400 Double rifle. Just couldn't handle the 400 grain regulation load of my double in the #1.
Regarding case fillers in the big Nitro, I used 1/2" foam backer rod, cut to about a 1" length, seated over the powder (65 grains R-15), and just very slightly compressed by the seated bullet. I researched fillers extensively, on AH, and in Grame Wrights, Shooting the British Double Rifle.
Both sources showed pressure testing and velocity data, comparing no filler, Dacron, and Foam Backer Rod (very similar to Kynoch wads.). Foam backer was the lowest pressure, least pressure variation, more consistent velocity, and in some rifles, potentially slightly more accurate. A larger data set is needed to prove accuracy benefits.
Given the minimal variation in pressure, and velocity, foam backer is a no brainer. And the fact that Kynoch has used them in some Nitro cases, and Butch Searcy used them, foam backer is the way to go.
