Tidy looking chassis youve got yourself

body work isnt always everything

For the tank i would highly recommend por-15
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weve used it before and its amazing stuff, we had a tank that if you held it up to the sky and looked through the filler pipe, it was like looking at a night sky full of stars, so we stuck some por-15 sealant in it and it was sorted

What do you plan to do with the body?
you can get the rubber gaiter from any dealer, a new one is around 50-60 quid i think.
Have you thought about moving the tanks and battery box on top of the chassis for belly clearance?
Keeping the standard engine?