Yes, you can switch between all three with no trouble. I didn't have to modify the stock fuel system.
And yeah, a 20# propane tank is equivalent to 5 gallons of gasoline from what I've heard. A 100# tank is actually about 29 gallons of gasoline. But the liquid in the tank boils off as its drawn from and the vapor is what is consumed. I've also been told a 20# tank would run a generator for 6-7 hours. Hopefully I won't need to use the generator on propane anytime soon though.
This is the company I bought my kit from: http://www.propanecarbs.com/small_engines.html
The particular kit I got is the tri-fuel kit.
Rob R., there is a load block on the outlet of the regulator mounted to the frame. You start by closing the load block the backing it off about three to four turns then you try to start it. It gets backed off until the generator runs smooth by 1/2 turn increments. Then you have to fine tune it with a meter that has a Hz setting. This is where I stopped because I ran out of propane. The tank I was using was pretty much empty cause it was from my grill. You can see the load block on the right side of the regulator in the last pic. It's closed because it was before I started adjusting it.