Mine has a "zero degree" tank where the neck of the tank with the multivalve sticks out the side of the spare wheel well, at about the 10 o'clock position as you look into the boot. It's well-protected by the rear axle and Panhard rod.

720mm diameter x 270mm height are the biggest dimensions that will fit while keeping the boot floor flat at the original height. Tank of those dimensions will be around 93L with side exit or 85L with middle exit, you should be able to fill to 80% of those capacities with LPG or 90% if the tank valve float arm is bent a bit. There is also a '4 hole' tank option, where the pipes can enter / exit the vehicle in just about any position you wish but they are more expensive and typically hold a couple of litres of gas less than the side exit full toroidal type.

I am very close to Frickley off-roading centre (otherwise known as Frickley pit tip) and have converted quite a few 4x4s including P38s that frequent the place. Like Gilbert said there will be no problem off-roading, and if you should ever roll an LPG converted vehicle the LPGl can't escape.

Working on a P38 at the moment, removing it's Piro LPG ECU and fitting a King, which involves changing the LPG wiring loom and interior switch, then we'll see how well the 10+ year old injectors and reducer work... parts have got better since then.

Simon