just to add to this,
-GK9*** is a package code used on all years.

-78-79 equipment labels will have code GK9, 76- 77's do not.

-wheels are 15x8's (all info i have are all 8inchers.)

-79's only avail w/chrome 5 slots

-78 as well but have heard of an orig owner
w/gold. all info i have is chrome

-all have code RU9- on equipment label for "Special wheel" code

-2 oranges avail sunrise and sunburst.

-76-77 equipment label/invoice states YE0- bucket seat package and KT1- black trim set. 78-9's have KB5- bucket seat and console

-all 76-77 have on equipment label/invoice GP2- Model Conversion. 78-9's do not.

-all have NF5-decal tailgate delete code.

all this info is from equipment labels, invoices and schedule notices.