Personally, with a recomendation like that - I would be looking for a new body guy.
um yeaaaa...
if it were going to be a race car, id say, your not done cutting yet?
it will come out stronger(at the welded parts anyways)
but to take a perfectly sound structure and alter it(and trust me, it will be altered)
thats a no-no.
is your guy familiar with chassis geometry? if hes not a chassis builder, walk away from the idea.
find someone with a proper sized tank.
theres lots of folks who dip full size cars.
im sure you can find someone able to accomodate it. if not, best of luck in any decision you make. id love to hear how that works out for ya.