Just found the leak on my '84 today and here is how I did it.  Go to the gas
station.  Get it filled up with gas.  Now stick your hand through the front
wheel well on the drivers side and the passenger's side and see if you feel
any wetness on the lines and fittings on top of the tank.  You can reach
almost all the fittings and o-rings this way and you can even replace some of
them without dropping the tank.  One of my hoses on the passenger side that
goes to the expansion tank is leaking (probably bad line) so I am going to
replace it and get some new clamps while I am at it.
Note:
If you have to replace the O-rings around the plastic fittings in the top of
the tank always pull the plastic fitting out of the o-ring, remove the o-ring,
then put the new o-ring in the hole and then plug the plastic fitting back
into the tank (I tried to put the o-ring on the fitting and shove them into
the hole as a unit for over a half an hour before I figured out the right way
to do it).
Have fun and try not to get any on ya.
Ken Wilford
John 3:16