I put a 6 gallon Harley Davidson Road King tank on mine all I had to really do was drill a hole and Jack_650 put a 5 gallon Softail tank on his.
Stock tank
Road King tank
If you go with the mod I did, you will have to relocate the speedometer which is not that hard to do, you will just need the tach cable from a GT 750 for it is a direct swap just longer.
Between these two threads you should get a pretty good idea on how to do the Road King tank mod.
http://suzukisavage.com/cgi-bin/YaBB.pl?num=1209434818/0http://suzukisavage.com/cgi-bin/YaBB.pl?num=1212029568/0