This might save someone else some grief. I have a Remington 700 that was a 300 Ultra Mag that I picked up just for the action. Most people can't shoot these things so you can find some good bargains out there. When I went to install my Near Manufacturing tapered base for a Remington long action it would not fit. I worked with Richard Near and found that a Weatherby Mark V base fit perfectly. I called Remington and they confirmed that the scope hole pattern is different on the actions they use for the Ultra Mags. So, in fact, Remington does have at least two different long actions.
If you haven't already tried one you would do yourself a favor by trying one of Richard's tapered scope bases. Besides the fact that they are of extremely high quality, Richard is great to work with. Here is his website: http://www.scopebases.com
Like I said I hope this saves someone a bunch of time and I also hope this is not a repeat of another post.