Toby....... If you do anything with that M70 you have, do 458 Winchester. I believe I recall it being 7mm Remington? If so, you do not want to go with a fatter case than what the magazine box is designed to work with, you will have endless issues with it retaining and feeding properly. You cannot successfully put a fat case in those M70s that are designed for that H&H based case, not without tons of work, and even then there are no 100% guarantees. Even with 458 Winchester there will be a few issues, but those can be sorted out. The mag box currently is set up to run those rat bullets designed in 7mm. Now you are putting a 450 gr + bullet in the nose, and the bolt dragging back over that doing it's best to snatch it out of the magazine on its way to the rear. The nose wants to come out of the magazine at that point, and some adjustments will have to be done to account for that. Its a fine line when it comes to feed/function, and you do not want to have a DGR that does not work properly, there would be no point in that..........
Brian and I at SSK looked at trying the HHBased M70s to build 500 MDM on, since the source of Winchester M70s in 300 RUM is very limited. It was a no go, Winchester did a hell of a lot of modifications to the RUM actions to get that fatter case to feed and function in a M70. Way more than is worth messing with and still most likely not going to get the results you want or desire in a DGR. This was confirmed by a few other gunsmiths as well. While the source of Win RUM based actions is short and few, and drying up, I had Montana do two actions for me designed for RUM, and they served the purpose extremely well, even building a Left Hand action, and hence a Left Hand 500 MDM for a good friend of mine. The Montana action did very well and when the source of Win M70 RUM actions completely dry up, then Montana can take the place of those.................