I have the old wider exit on my tank as I have an 83.5. There is a grommet
you can get that enables you to add a narrower filler tube from a later
model that has the "floaty ball" to your older tank.  I have a video at
http://www.youtube.com/watch?v=J4X99U2wS_U and at about 4:20 you will see
the grommet.

Thanks,
Bryan