I have used H-4350 in my 404 Ruger African (same case with a 0.423" bullet), but prefer Rl-17. The latter's smaller granule size and slower burning rate allows me to use more powder and obtain greater MVs.
Although I had my 375 Hawkeye Alaskan also re-bored to 0.423" I haven't taken it to the range yet. It should prove interesting with 400 grain bullets and a full load of RL-17. In the 23" African my top RL-17 load has a MV of 2416fps with the Barnes 400 grain Banded Solid.