WOW!
That's a really long trip... I'm jealous; it'll be a lot of years before I have the time to do anything like that.
A larger rear tire might make the bike ride smoother and slightly more comfortable. The 140/90/15 rear tire that these guys are recommending is just the size of the tire. You can find a lot of different brands and types out there. Also a lot of opinions on here about which tires are best. Look for a good long distance touring tire.
I believe that size motorcycle tire should fit right to your rim like normal. When you're talking about the post that discusses the turkey carver, I believe you are referring to the thread about using a MUCH larger car tire on the Savage. I definitely wouldn't recommend that for your needs, and if you read that whole thread, you'll see it's generally NOT recommended at all.
I wouldn't worry about changing the carburetor either. There are a few simple mods that you can do to the stock carb that will help performance, but the more you do to make the bike faster, the more you're going to lower your gas mileage. In your case, if you're happy with the stock bike's performance, which should be perfectly reasonable for that kind of traveling, I think you're best off leaving it alone. It'll also be easier if you do end up having problems on the road if you have a more-or less stock bike and need to find a mechanic to help you out in a pinch.
I'd be focused more on your shocks, springs, seat, and handlebars. All the things that are going to make you more comfortable. If your bike is older, I'd also be sure your steering bearings and other bearings are in good shape; wheel bearings, swingarm bushings etc...
You're going to be putting a lot of miles on that bike; a lot will depend on how old your Savage is and what kind of mechanical shape it's in. What's the bike you are starting out with?
Most importantly, if your Savage doesn't need one of these yet, it probably will before your trip is done. Might as well do it before you leave!!
http://suzukisavage.com/cgi-bin/YaBB.pl?num=1180206459