That link should be
http://www.luvtruck.com/Information.htm
As for narrowing down the year, you can pull that info from the VIN code if you have it, I think the assembly date is the 6th digit, though it might show a 6 for a 77 model year and so forth. From your description I would guess that it would be a 75-77 unless something obvious marks it as older like tail lights under the tailgate. If it has the narrow front bumper and tail lights up the corners of the bed, its a 75. Wide front bumper came in 76. I wrote up a rundown of this info a while back at
http://forums.luvtruck.com/viewtopic.php?p=13553
The year to series (for picking a diagram from the link above) goes something like this:
72 - series 1
73 - series 2
74 - series 3
75 - series 4
76 - series 5
77 - series 6
78 - series 8
79 - series 9
80 - series 10
81 - series 11
82 - series 12