You would not want a 7mm mag to build a 308. Yes, the bolt face is too large but it's also a long action. You will want to stick with a short action to build any rifle based on the 308 cartridge.
If you want to build and extremely accurate 700, a lot of work needs to be done to the bolt. I usually just order the .703 bolt from here http://www.pacifictoolandgauge.com/products/parts/remington.htm and have the smith like Rosco at http://www.louisianaprecisionrifles.com/ ream the receiver to .705. Doing this, you will not use you're bolt anyway. There are a number of other things done with the bolt also to get the most from you're rifle. When they ream the receiver, they will true and square the threads using the ream as a center so everything is perfectly square. When things a done right by a good smith and install a quality barrel, you have the major portion of a tack driver. A quality pillar post bedding and good stock and you will have one great shooting rifle.
Another option is you can order David Tubb's bolt http://www.davidtubb.com/700_bolt.html and have the smith install it. I've also used these in a couple of rifles with great succes but I've heard some say they had minor timing problems with them, I didn't. They mainly do not look no where near as nice as the PT&G bolts and with the PT&G, you can get it snugged up to a perfect .002" clearance.
I might also suggest finding the older 700, mid - late 60's etc, they have the better trigger. I'm not sure what year those go up but I pick up the 60's models in the lenght action I want anytime I want to build another.
If you're not sure, you can go here for year models. http://www.remingtonsociety.com/rsa/questions/barrelcodes
If you want to build and extremely accurate 700, a lot of work needs to be done to the bolt. I usually just order the .703 bolt from here http://www.pacifictoolandgauge.com/products/parts/remington.htm and have the smith like Rosco at http://www.louisianaprecisionrifles.com/ ream the receiver to .705. Doing this, you will not use you're bolt anyway. There are a number of other things done with the bolt also to get the most from you're rifle. When they ream the receiver, they will true and square the threads using the ream as a center so everything is perfectly square. When things a done right by a good smith and install a quality barrel, you have the major portion of a tack driver. A quality pillar post bedding and good stock and you will have one great shooting rifle.
Another option is you can order David Tubb's bolt http://www.davidtubb.com/700_bolt.html and have the smith install it. I've also used these in a couple of rifles with great succes but I've heard some say they had minor timing problems with them, I didn't. They mainly do not look no where near as nice as the PT&G bolts and with the PT&G, you can get it snugged up to a perfect .002" clearance.
I might also suggest finding the older 700, mid - late 60's etc, they have the better trigger. I'm not sure what year those go up but I pick up the 60's models in the lenght action I want anytime I want to build another.
If you're not sure, you can go here for year models. http://www.remingtonsociety.com/rsa/questions/barrelcodes
