vacuum

Ok, my 77 Ranchero has a 351c in it... it has a Dizzy out of a 77 351m
question, where should I be getting vacuum for the dist. from? right now
its coming off one of the vacuum ports controlled by the water temp. I know there
are 2 groups of 3 each, the one I'm using it from the lower group and
using the end port.
Best
sorry that this is not a (carb) only question, hope someone can answer)
ps: what is going on with the 351 cleveland web site, I could not
get into it...I am a registered user.