Paul,
Welcome to N.E , your choice of .416Rigby in a No.1 is a desicion I had to make not long ago but I chose the .450/.400 now offered by Ruger my reasons where that the 450/400 has a rimmed case which aids in extraction, traditional cartridge for single shot rifles where as the 416Rigby is essentially a bolt rifle cartridge, also the 450/400 feels better and is more balanced than the 416Rigby which I found to be to heavy in the barrel, I would go for the 450/400 as it shoots a 400g bullet as does the 416Rigby but if happy with the 416Rigby go for it!

steve