I just ran your question through www.ammoguide.com and it said that it will take in your action length the 450 Ackley Magnum but it did not show the 450 Rigby. As for the rifle, Winchester I think would be a better choice.
Have a look on www.usedguns.com.au just now, there is a Custom Sako L61-R .450 Ackley Magnum for$2450 Including dies and brass. You couldn't buid it for that.
Yeah I've already seen it but I already know where there is a Winchester S/S with cases and dies for considerably less, albeit without a nice timber stock. I will have a Winchester Safari Express take off stock I could put on it though.