Both calibres are popular over here, with the 404 seeming to be on the up for use on big stuff at the moment.
Personally i would steer clear of a hot loads in the Rigby. A 6000 foot pound thing kicks like a mule and there is a high risk of bullet failure on heavy game at close range. Better to load the Rigby at around its original operating levels (400gr at say 2400fps) and use another, lighter weapon for longer range applications (The .338WM is a good choice for that).
That having been said there are not a lot of places over here where one would need to take shots in excess of 200m, and even that would be considered a long shot except in the desert regions.
Yes and the 404 J is loaded up from historical levels. Also look at the rifle in terms of quality and how well you shoot it. This morning I took apart a "classic" Model 70 to float the barrel and I am a bit disappointed with worksmanship and the blob of epoxy material that acts as pillar bedding, stamped metal parts and an "alloy" trigger guard.