Kevin
Might want to think about 458lott instead of Rigby you will find it quite a bit cheaper to shoot.(can use 45/70 bullets hornady factory loads for it so dies and brass are quite a bit cheaper) I am building a 470 capstick on an enfield action now. Learning as I go.
You have to build a new mag box and if it tucks up into the action I believe it must be opened a touch. I made my box like a mauser so it doesnt tuck in and I dont think ill have to open it up if I do it will be very little. The action is 3.710 inches the 458lott is 3.6 col.
Im not sure but for the Rigby you might need the P14 bolt. The 1917 enfield is the action for the big stuff Im told. Looking for another myself.
Dean