As I am from Europe myself and the cars there are run on LPG since I can remember and that is even earlier then the 70's.
We ran several cars there from 0 miles on the odometer and every mileage we wanted to run it on and it is even on every type of gas up to diesel that we ran it as diesel doesn't work with LPG.
But as LPG is although it is a liquid ran drier then gas it is recommended when you got a installation put in you would run it every three fillups at least on gas instead of LPG for a few 100 miles just to keep the engine from getting to dry
The problem you might be referring to would be the freezing of the conversing unit that could only occur when you were to hastely switching from LPG to gas as that unit converted both to run in the engine to prevent that you switch from either to a neutral stand run the engine until it started to sputter and then turn it to the gas you were switching to to maintain the engine to keep running.
It is already proven to burn cleaner than gas as it is a dryer burning fuel.
The age of the car is as you mentioned not an issue.
Every car is suitable to convert to LPG it is a myth that the conversion wouldn't be possible for surtain cars,it was in Europe not the type of car that was the issue for conversion but the amount of miles you were running to make it a LPG running car (salesman or any occupation that would run you around the country)
The only setback in Europe was that for even while thinking of the environment you were punished by having a higher road tax tallied on your car.
What you consider a drawback actually isn't as it would have you run on regular gas to get there so it would help you keeping the engine a bit lubricated.
Hoping to be of help with this answer.
A flying Dutchman