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I'm having a play modifying my mpi sports pack mini and want to fit a right hand tank . It's just occurred to me that the left hand tank has a internal electric fuel pump . The mpi tank doesn't have a breather or vented cap because of it using a pressurised system . Any idears on connecting the left tank to the right bearing in mind that the left tank doesn't have the same connection / outlet tube on the bottom of the tank as the original mini. Or would it be just the case of welding in a tube to the mpi tank bottom so the right hand tank can drain into it as per original set up . Also I would have thought if the mpi tank doesn't have a breather , would it be ok to block the breather on the right hand tank and cap ? . With the vents blocked in the right hand tank , would this stop the fuel draining into the left tank ? .... Ken

Are you sure it doesent have a breather, I thought they had a rigid plastic pipe at the top leading into a non return valve?

The right hand will need some form of breather otherwise, as it empties, it would create a vacuum and stop the fuel flow out.

I'm not an expert and am happy to be corrected but I'm fitting the MPI system to my current project.
The MPI isn't a pressurised system as such but it is 'sealed'. The breather pipe from the top goes through a gravity valve that prevents fuel escaping should the car turn over. Then it's connected to a line to the engine bay where it enters a charcoal canister that absorbs the fumes when the car is at rest. They are fed into the engine when it's running (controlled by the ecu).
So I would think that you could tap into that line and connect the RH tank breather too. You'd need to use an unvented filler cap like the original.
I can't see then why a balance pipe similar to the Cooper S would not work fine but if you're proposing to weld a tube onto a used fuel tank you're braver than me.

When I say 'sealed' I mean sealed against the emission of petrol fumes. The purpose of the canister is to allow air in and out (to compensate for fuel level changes and temperature variation) while capturing the fumes.