Contact XLR Ind. to be sure your rifle, a long action stagger feed, will function in their chassis as a 300 win mag. We know the bolt spacing is correct but the magazine port on the bottom is in a slightly different location than the center feed mag port. The concern being feeding without problem. I think it will work but I would make sure.

I believe both the standard long action bolt head and the Magnum long action bolt head are the same lug length at .500' so your firing pin is good. As Dave said the only thing is the older firing pin may be slightly larger so if you get a new bolt head you may have to drill it slightly oversize. That will require some a drill bit that falls into the letter system. I would measure the portion of the pin that goes through the bolt head. Not the area of the cross pin. Go too large and you start cratering or even piecing the primer. It is better to get factory matching parts. The best idea if your pin does not fit your new bolt head would be to have this guy fix you up. https://deshind.com/ . This is if you pin does not fit your bolt head. Your cross pin should be good if you do not change your cross pin or firing pin.