Basically all you will be doing is adding a LPG carb, a fuel pressure regulator and a LP tank.
It runs completely separate from the petrol system, you can run completely out of petrol and run on LPG or vise versa.
You will need to add a fuel pump shut off switch for your petrol fuel pump so that when you run on LPG you shut off the petrol pump and turn on the LPG.
Keep in mind that you will loose a good 10% or more in MPG and horse power while running on LPG and you might throw a check engine light for a O2 code.
Also think about where you are going to get your tank filled.
While in high school a friend and I ran out of gas while 2 tracking in his '76 LaBaron with the slant 6.
We took a propane torch that was in his truck, turned the gas on and stuck it in the carb, we drove another mile or so on that and then "borrowed" gas from a farmer.