Wow, It's amazing see the locations of the responders on this thread. I guess we've got EMEA covered.
Here's an add-on tank kit sold through safeair. I can vouch that I'm happy with other stuff I've purchased from them...
http://www.safeair1.com/HWA/er_RV6.htm
I'm not installing tip tanks, at least from the start. I think I'd need a personal long range bio tank to take advantage of an extra fuel tank. If you went with fuel injection and electronic ignition, the possibilities for more efficient fuel burn might also be a good way to add range.
good luck.
Don