This work can be done. It involves opening the bolt face and then soldering a ring into it to cover what is left of the extractor slot, and then boring again to except the magnum. An M-16 or Mini 16 extractor is then installed. requires machining and a bit of fitting, sometimes. A factory barrel is rarely worth the expense to re-chamber, so a custom barrel is recommended if you want it to "shoot" after sinking $$$$ into the project. Lastly, feeding should be addressed. It is not impossible and there's really no need to go to center feed mag set-up. Just takes some know-how to achieve reliable and 'slick' feeding by someone who knows and understands the feed cycle. You're gonna' need a magnum follower for the Remington, also. In the long run, I feel it is better to start off with a magnum action for a magnum rifle. The kind of work you're looking at doesn't come cheap, but if you find someone who will do this work cheap, run the other way. I've seen plenty of half-a$$ed jobs like this 'conversion' that didn't work right, but they were 'cheap'.