Re: Marlin MR7
I'm sure just about any good gunsmith could fit just about any barrel you would want but it probably wouldn't be worth the extra cost. The MR7's cost would probably be less than what a good barrel alone would cost. Not to mention that if you wanted to accessorize or modify you rifle your selection of aftermarket parts such as stocks, scope rails are pretty much non-existent for the rifle.
The 270 is a great cartridge and as long as the gun is accurate I would say to just shoot it as is. Get your practice with what you have now and learn as you go. A 270 can do a really decent job to a good distance, as best as I understand the biggest thing it lacks is a good selection of match bullets although there are some. As you get more into thing I would look and read things on this site. I've learned a ton just reading. You can get a better idea of what you want and like then as you upgrade equipment.
Good Luck
JMC