There is normally a small T-fitting or something like that coming out of the manifold between the back of the carb and front of the distributor. Yours may be plugged since it had manual brakes originally. That is the perfect kind of part to grab from a member parting a truck on the parts board.
Post 5 shows the plugged hole in the manifold:
http://67-72chevytrucks.com/vboard/s...d.php?t=589077
Summit sells the fittings too:
https://www.summitracing.com/search?...cuum%20fitting