Sweet!  Was thinking about doing the same thing.  Really hard to go anywhere when you always calculating whether you are going to make it to the next stop.  I like to ride up here in the Sierra Nevada mountains, but the gas stations are few and far between.  The tour tank option is a really cheap and easy solution to the small tank problem.

I was thinking of a 3.5 or even a 4.5 tank.  Was going to put a quick disconnect couple on it so I can remove it easily.  Even figured that you should be able to put the tank inside one of the Buffalo Leather Barrel Bags they sell on ebay for $10.  (I bought the 7 pc kit).  It's perfect for "tour" use because you can put the tank in the bag and the bag comes with plenty of straps to secure it (2 straps for the sissy bar and 2 for tie downs).  I was just going to set it on top of the rear seat and strap it down.  A 3.5 full tank should way about 25 lbs, 4.6 about 31 lbs.  Should be just fine.

This is what I'm talking about:
http://cgi.ebay.com/ebaymotors/LEATHER-MOTORCYCLE-BARREL-BAG-LUGGAGE-SADDLE-B...
You can hide the fact that it's a fuel tank.  Would just look like regular luggage

Curious to know how you plan to run the fuel line T and petcock.  Let me know how it turns out (pics please!).