If the vacuum advance stayed active during wide open throttle, you would have too much advance at higher rpm. For pure performance, you don't need any vacuum advance.
With ported vacuum, you don't have any advance at wide open, either.

To boil it down, it ends up working very much the same but you have more advance at idle, it runs cooler, and is a little more torquey down low - all good things for a street vehicle.

To stay on subject, if Marty doesn't have "correct" ported vacuum available, manifold vacuum absolutely is an option.

 

I looked at the link for the modification to the Holley 94 to get the correct vacuum to an updated distributor. It looks pretty straight forward. I'll be looking into the possibility of getting a newer distributor but for now I'm pretty happy to have my truck working properly.
Thanks again for all the replies!

 

Like Charlie, I run manifold vacuum too. No problems. Both our trucks run great. I happen to have 3 carbs though. Dennis

 

I have a 318” Y-Block with three Strombergs. Not being able to find a rare Stromberg with ported vacuum and not willing to drill holes in one of my original bases (I run the bigger 48 base in the center) I too run manifold vacuum. I should also mention I have a Ignitor III with multi-spark.

I did a lot of research on manifold vs ported vacuum advance. I came across many articles pro and con. Here is one that makes the most sense and is well written; Ported vs. Manifold Vacuum

I also agree with the part that the high dollar mechanical only distributors have no place on a street machine.

Please no snarky remarks because it is on a Chevelle Site.

 

Sorry for a late reply, I've been off on other projects. Thank you for the link to that article, that was an interesting theory and makes sense in many ways. I'm going to give this a try as soon as I replace my vacuum unit which isn't working. The original owner had just put a new Holley 4V on just before I got it, it's a small cfm off road model that has both a ported and a manifold vacuum port. I'm not familiar with these models so this is the first one I've seen set up this way but will make it easy to test.