I was undecided on manifold or port vacuum and i was about to buy a distributor from one of 2 people that set them up on a machine to your combo. I ended up with the one set up for port vacuum but i wish i would have went with the manifold vacuum because the idle stinks like hell with the 284/484 cam and the manifold vacuum may have cleaned that up a bit..
there's no reason why you can't try manifold vacuum if you want. i'm a port vacuum guy, but if engine vacuum is fairly low manifold vacuum use may be the best choice. i have to wonder if your distributor is set-up for proper initial and low rpm centrifugal timing. the 284/484 is fairly aggressive with the 108lsa and a hydraulic tappet doesn't help much.
