Must admit I have not heard about the petrol tank replacement types. They would be horrendously expensive anyway because of the proof testing involved. (It's not proof testing but I can't recall the correct wording, you get my meaning.) It's relatively easy on some 4x4s where the petrol tank can be replaced with, say, a 70 Litre LPG cylinder and a 35 Litre petrol reserve. But that's not the same thing.

Had I bought a Tourer it would still have gone on LPG as would any car that I have, even a diesel. (ugh!)

I would have probably looked to see what a couple of smaller cylinders looked like mounted longitudinally on either side, leaving the centre clear or raised the floor. It's always a compromise anyway.