Terry,
The dipstick tube on a GMC 305A measures 14 inches to the engine block with the 6 quart oil pan. To verify your oil pan size observe your oil drain plug. Your plug will be on the back of the pan angling downward with the 6 quart pan. Oil drain plugs on 8 quart pans exit the bottom center of the pan.
Have a great day!
-Jeannie
6066 GMC Club