The full vacuum thing is quite a controversy. Folks have "made it work" either way. The whole system takes on completely different strategy one way or the other
The big block SS Chevelles came from the factory with manifold vacuum on the distributor. I didn't know that until I helped Matt do the restoration on my friend's 1970 LS5 car, but it's right there in the GM manual. We tried it both ways. Car ran like a slug with ported vacuum and ran like a banshee on manifold vacuum. I timed it accordingly for both ways. It was strange.
