79 and going strong...... I wanna be like you! Thank you for the kind advice.I will be 79 next month and still enjoy the big bores. I was also extremely myopic until I had Excimer surgery years ago. Since then I have experienced tears in both retinas. The left eye when hit by a softball and the right eye after getting too close to the scope on a 300 Win Mag. Since the tears I still shoot my 416 Rigby, 450 NE, and 470 NE with no issues other than at my age I do not agree with open sights. I have RMRs on my double rifles.
With respect to calibers, with the modern bullets (Woodleigh Hydros) I would believe the 450/400 to adequate for Elephant and as the client you have the PH for backup. I also second the choice of a Heym 89B. I love mine in 450NE and I have been able to handload reduced loads that regulate.
Hope you can get to Dallas to shoot the Heyms and I am sure you will enjoy your time with Chris.
VC offers it as wellOf the ones you listed the 450/400NE 3" would be my first choice with the 470NE a close second for the same reasons so many others have stated.
However I would like to offer up one that isn't listed...the 416/500NE. This cartridge is of much more recent design, I believe 1996. It was made to handle modern powders and bullets. There are several bullet offerings when it comes to the .416 caliber for reloading. It has a tapered design to feed easily in a double rifle.
Lastly are the ballistics. Compared to the 450/400NE, it's not even close. The 416/500 produces 5000 lb-ft of energy at 2400 fps making it perfect for elephant or buffalo. Heym and Krieghoff both offer this cartridge in their doubles.