Funny thing about the older plastic tanks: ethanol in the gas makes the fuel evaporate right through the tank walls at a noticeable rate. ARB had to increase its evap emission rates in the EMFAC model at one point because of that. Modern plastic tanks (since about the early 1990s) resist that, but those old Chrysler tanks will do it. For that matter, so will the plastic tanks on old lawn mowers and other powered landscape/construction tools (buy electric now - batteries have improved a lot in recent years).