I have been looking at this same project. I'm going with either 460 wby of 500 A-square which is based on the 460 wby case. I'm using the 416 rigby CZ action for $500- $600 . CZ sells straight actions so you don't have to find a donor rifle.
If this is going to be a true DGR then you need to consider a action with control round feed. They are knowed for their reliabilty. Weatherby is push feed just like Rem 700 so both are out for DRG. Besides the WBY is ledgen for breaking on hunts.
As far as ruger it will cost you $1000+ for a action, and ruger is the one rifle action that I won't own (personal Preference).
Good Luck Kevin Weaver of Weaver Rifles has a 460 wby reamer and has built several of them on CZ actions.