If you're not riding with two people on the bike you can get a lot of gear on a 650. Check out the picture of my Savage fully loaded on a 4,400 mile trip to Miami and back. Loaded like that I still could run 70-75 mph and keep up with the Sturgis crowd - until I couldn't take the noise.
http://bikepics.com/pictures/1162560/As to converting for the oversize tank, there's lots to do but none of it's terribly difficult. I had to move the starting solenoid thing and make a longer cable for it, reroute some wiring, move the electronic doo-hicky that's on the spark plug wire, adapt the front of the seat mount (I have a different seat as well) and a ton of other trial-and-error things. And there's really no room for the chrome on the top of the motor on my bike with the different tank.
http://bikepics.com/pictures/869173/http://bikepics.com/pictures/869170/I got the tour tank first and then picked up the Harley tank with a dent from the dealer for $50.00. It's nice to have the 400+ mile range between fill-ups if I'm out in the boonies or want to wait for a better price. That being said, after all the work, skinned knuckles and trips to the hardware store, I might think differently a second time around and just go with the tour tank and act happy with 250 or so miles between fuel stops. Then again, I do get asked "What the hell'o kind of bike is that you're riding" every now and then, so . . .
Jack