1970 DUSTER FUEL TANK

Hi, I'm getting ready to order a new fuel tank and hardware for my 1970 Duster that has 225 slant 6 in it but I will be converting to a 360 in near future. Question is when I'm looking for fuel tanks online there are 16 gallon and 18 gallon tank sizes in 1970 and also with or without vent nipples. Was the 18 gallon tank an option and will it fit in my Duster?