Ruger has 2 action lengths in thier standard 77's, The factory 375 H&H, 458 Lott's are built on the RSM (Ruger Safrai Magnum) action. This action is much larger than the standard 77 MKI and 77MKII. Most of the 458 Win. Mags I have seen have been on the MKI with tang safety. The rechamber is straightforward, the mag box, rails and action must be opened up to use the Lott. It can be done but will not be cheap. A guy over on accuratereloading.com did one several years ago and I am sure many others have.