I have an OMVL multi-point system on mine, with underslung LPG tank and small replacement petrol tank. The stock petrol tank was plastic, these two are steel and I have fitted a fuel tank skid plate over them both so no problem off-road. This setup reduces clearance by about 2 inches closest to the axle and the loss tapers off towards the rear. With the skid plate and the lift it doesn't cause a problem off-road.
I fitted my LPG kit and had it certified by a company now out-of business. The kit cost around £1350 and about £100 on setting it up and certifying it.
I've done nearly 50k miles on my system with no problems and would definitely recommend it.