I looked and since I do have the right hand filler that was my thought.  But, when I looked in the trunk it's just screwed into fender, no actual hole.

Oh well it was just a thought.

If interested, I have a right hand tank with the neck relocated to the inside so it won’t require cutting a hole in the body. Completely refinished and available. If interested drop me a note on: mebmini at aol.com

You can buy dummy fuel necks as a styling mod - I've got one but never got round to fitting it.

From 25+ years of long distance trips in Minis, I can assure you that the driver needs refilling or emptying long before the car does...

With the larger 7½ gallon tank you have 250+ miles and over four hours of range.  If you can go that long without needing a wee or sustenance then well done.

I have a right hand tank (actually I might have two of them) that I might have the guts to try and install this summer. One is a metal OEM tank (location of which I can't recall right now) and the other is a plastic replica that I got from a vendor in Australia (or New Zealand, can't recall which, back when shipping costs were palatable.) That's the one I'll install.

But rather than have the main and second tank free-flow into each other, I think I would like to have a electronically controlled valve to control the flow (like the big U.S.A. pickups with twin tanks.) 

I like being able to cruise for more than two or three hours at a time at freeway speeds before having to refill, though in reality it's probably a good idea to stop every 2-3 hours. I'd feel better going through the less inhabited countryside of North America with the extra fuel tank though.

I just have to have the guts to cut the metal for the filler neck, but since I've been doing rust repairs on the right rear corner why not just cut it. At worst, I have a replacement right rear corner to go in if I mess it up...

I am.

I've thought about it, but $722.98 still buys a lot of gas for a Mini, even at today's prices. It is a bit of hassle to fill up so frequently, but even those take 5 minutes or less. 

Thanks guys.  I was wondering if it would be worth it?  Is anyone using the twin tanks?

The two tanks get tied together with an interconnecting metal tube that runs between the two. Both tanks drain together as one so the gauge reads the total fuel level the same as it would for a single tank.

Classic Mini Cooper S 5.5 Imperial Gallon Right Ha (minimania.com)
 

I have a 93 MkVI Mini.  It has a gas cap on the right side but obviously there's not tank.  Just wondering?  They do sell a tank for that side, only 6 or so gallons but??  

Anyone have one?  Then does the gas gage read the balance of the two tanks?  I changed my final drive to 3:1 vs the stock 3:4 which helps but still I wonder???