Thanks to all for the advice and observations so far.
One final thing I need to sort is properly securing the tank in place. It's been pointed out a couple of times that centre of the turret is not a permanent piece of the rest of the wheel well, and is bonded in.
Thing is I'm struggling to visualise a solution - so want people's observations of what is acceptable for a safety inspection. My first thought was in fact to install a pair of sold bars above the tank - to prevent it moving forward/upward in the event of a crash, as below:
Straps would be say 2mm thick, 20mm wide flat steel bar, securely bolted through the vehicle body at both ends - and formed slightly so they sit in to the wheel well - with material between them and the tank to prevent rubbing.
If that's not acceptable, does anyone have any examples of how I might do this from underneath? Would it be a case of fabricating some flat spreaders and using them place of the washers that came with the tank - so that the spread the load betwee the bonded and permanent sections of the wheel well - something like this (red lines indicate the spreaders - one slightly out of shot):
Come to think of it - my preference would be to use something like the spreaders, as the straps option is likely to be a pain, in terms of access for the bolts at the rear of the wheel well (likely obscured by diff & driveshafts.