I'm not overly keen on bashing the fuel tank into shape so that it clears the bonnet , seems a bit brutal , also not keen on cutting the tank and welding it back together
(My welding skills are "agricultural " ) , I was thinking of mounting a tank on the rear seat shelf , anybody done this and did you get an off the shelf tank ?   

Quite common to fit cylindrical alloy tanks in the rear. I don't like them myself, they tend to be quite small, seldom have a guage sender and can be smelly, plus I just don't like that much petrol sloshing around behind my head and that close to a hot engine.

A little bit of gentle panel beating on the tank corners is tried and tested and had worked for almost 50 years now on buggies.

Another plus point for leaving the tank at the front is that buggies ride much better with the weight of the fuel being over the front axle.

also not keen on cutting the tank and welding it back together
(My welding skills are "agricultural " ) , I was thinking of mounting a tank on the rear seat shelf , anybody done this and did you get an off the shelf tank ?

If you are moving to a central fuel filler you are going to have to get some welding done.

Best option is to take it to a radiator repair place, they have the skills to weld and will also pressure test it afterwards.

just fit a fuel cell  as used in racecars

Alan has a new contact up north for custom made tanks where you can send in a cardboard one you make to fit as a pattern

The company I use is called .
Compbrake motorsport in Wigan
Tel.01744 895888 (Clint is his name)
Basically I made a cardboard tank to the size I wanted and then took measurements from it ... I did visit him with my drawing / measurements but you can email . The tank he made for me is 3mm thick aluminium and is spot  on . Fitted a treat  and only took 2 weeks to be done ... I will use him again