VampireTigger wrote:If the old one can be saved I probably will get it sorted so there is a spare available
It would be worth looking at. I had a brief chat with a fuel tank company at the NEC a few years ago about making a custom tank to roughly the same shape (I imagine it would be more square than the proper one) and they were talking about £750-odd.
That said, before I spent that I'd buy a new MGB tank and get the inlets and outlets swapped over. The outlet on those is just alongside where the exhaust comes out on a Viva, but otherwise looks very similar. I haven't measured it, though, so don't rush out and get one on my say-so. Good thing is - new ones are quite cheap, and if they're new, you can get the alterations welded without worrying about vapour.