I've fitted the under car LPG (twin tanks) to my '88 Rangie. They have a combined capacity of 80 litres (the AFL cuts out at 70, but I can get an extra 10 in slowly for a long trip).
I've got a rear quarter panel mounted tank which uses the original petrol filler. This tank holds about 40 litres. Then I've got a couple of sill tanks which fill in under the wheel arches and I've mounted an early Rangie fuel pump with an electric 3 way switch to transfer fuel to the main (quater panel) tank. Each sill tank holds 33 litres (depending on the angle the Rangie is parked at when filling).

This gives me 80 litres LPG and 100 litres ULP. On highway travel that equates to :
80 x 4.5km per litre = 360 + 100 x 7km per litre = 700
Total 1060 km Range -10% safety margin is 950km between refills for highway touring.

I picked up the tanks 2nd hand in the Trading Post and the LPG system from a Range Rover wrecker. No body wanted to fit the LPG because it's a long and tedious job with no profit margin. Eventually I got a mechanic who did it at mates rates, but the teething problems weren't worth the savings.

If you plan it out right you can extend your range without encroaching on your cargo capacity.