I thought someone would have responded by now.
Here is a thread on grounded the trucks. It covers almost all the grounding spots on them. I posted it because this is one of the more common questions in the electrical section.
http://67-72chevytrucks.com/vboard/s...d.php?t=422234
Post 28 on page 2 will apply to your question.
If the cab, frame, and engine are well grounded You should only need to ground the gauge cluster to the cab, and anywhere there is a good bare spot should suffice.