No disrespect to anyone.
Sarduy - it's a lot easier than these guys are making it. Cheaper, too.
You can buy a used 308 barrel on Gunbroker for around 50 bucks. Mine cost $57 with shipping. Brand new factory 700 barrels are also available at various prices. Custom builders buy new rifles in whatever caliber, pull off the barrel and sell them cheap. They are up for auction daily.
Unless you're already good at gunsmithing, you probably cannot get the existing barrel off the action, nor can you check the new barrel to see if it headspaces correctly on your action. If you have that skill, you can save a hundred bucks or so. I did it, but I've been doing that stuff at home for about 10 years, and I own the barrel vice, action wrench and headspace gauges.
My local gunsmith will swap barrels on a 700 for $100, including reaming the chamber if the headspace is too tight. He also tries to push you into $150 to reblue the rifle (mostly because he mars the bluing while removing/installing and he wants to cover his marks). But if you're not picky on prettiness, you don't have to reblue. The finish isn't ruined, only marred sometimes. I didn't hurt mine at all.
I made NO magazine modifications to my long action. It takes shorter cartridges the same as the long ones, and feeds them perfectly into the chamber. I asked my local gunsmith if mag modifications were needed and his reply was, "Who told you that?" Then he laughed. Lotsa stuff gets said every day, but often truth is much simpler. Do you have a friendly local gunsmith? Stop by with a Coke or Pepsi and talk to him for two or three minutes. I bet he's a nice guy when you arrive offering a small gift and only need 3 minutes of his time. But don't take up half his afternoon, he's trying to make a living in that shop.