There are two types of tank and the transition year was 1979. Most likely you are after a under seat mounted one but it would help if you confirm. If it is an under seat one you need to say whether it is a LHD or RHD as I'm sure that they are interchangeable.
Other than that, welcome and please take a few minutes to introduce yourself, if you haven't already done so (and I've missed it)
Regards,
Rodger