Here's a picture that shows the vacuum tree at the upper right corner of the pic on my 69 . There are 3 ports, but only 2 are clearly visible. The third, to the transmission, is on the opposite side of the bottom fitting. Power brakes were added by the dealer as a condition of original sale when the truck was new. I'm not sure this resembles how the factory did it. This setup provides a vacuum source for the brakes, A/C and transmission (TH400). Here's a link to the fittings sold by Corvette Central.
https://www.corvettecentral.com/sear...tting&count=18