Most car warranties including the Ford Vehicle Warranty specifically excludes any defect in any vehicle component that is caused by or is attributable to, a modification to a vehicle.
A warranty can't be "voided" but Ford can "reserve" (I think the word is) the right to provide warranty provisions to fix/repair a defect where it believes that a modification made to the original car caused the defect.
So .... things like fitting the car with LPG aftermarket, lowering, exhaust systems, bigger/fatter wheel and tyre combinations are most definitely modifications.
Say for instance, a complete 3 inch exhaust system which affects clearance underbody melts or damages a wire or something. Ford could reasonably say that it is in no way responsible and direct you to your insurer or the provider of the exhaust.
I guess you may not have a problem with a reputable modifier like Herrod's --- but it still DOES place you in an area of uncertainty to get something fixed under "warranty" that ultimately could require legal wrangling before it gets sorted out .... can you smell the $$$$'s burning away yet?????
Basically just be careful and understand what you could be getting yourself in for ...... before you do it. Remember ignorance is not a defence in the eyes of the law.