Whats up Doc.,
I have a mitsi L200 for the past few years with a 350ltr. upright tank in the back. Its been strapped in with one heavy duty ratchet strap fixed at either end of the S hook ends to D ring points on the upper sides of the rear cab.
Tank has never ever budged - whether empty or full.
Its placed right up against the rear bulkhead and is a good snug fit between the wheel arches. I wouldn't drill hol;es in the floor though.
You could alternatively place tank tight to rear bulkhead and between wheel arches and put a cross member brace of mild flat steel accross it and in turn fix this brace to frame / points on the ' walls ' of internal cab. ( obviously not going thru. ! )
Ray