LMC has a 64-66 decal set that shows an optional vacuum or air pressure gauge (see attached picture).
The original optional gauges will be hard to find.

Also there is discussion of a dash restoration at other forum:
http://67-72chevytrucks.com/vboard/showthread.php?t=562983
Attachments
Decals and Vac 01.jpg (61.33 KB, 35 downloads)

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."