On the B/RB engines, the valley pan IS the intake manifold gasket. The 440 3x2 engine was where the additional gaskets were first used, being it was an aluminum Edelbrock intake OEM. I have a Torker intake on my '67 Newport 383. I thought it was getting a vacuum leak from the intake, so I pulled it all apart, added the additional gaskets, and it seemed better, but there wasn't an intake leak to start with. But I now KNOW everything is sealed well, there.
A bad vacuum advance, from experience, will cause a big drop in mpg with little performance loss. Other than possible throttle response decreases. When the power brake booster diaphragm starts to leak, a big "miss" will happen when you press the brake pedal. Either of these might cause smaller vacuum leaks, but not terribly bad by themselves, from experience. Vacuum advances are available new, from replacement vendors. I found that recently when researching another post about them. Might not be at the local auto supply, but still around (i.e., rockauto.com) without having to go to specialty vendors. But as you've got the MP distributor, you're fine in that respect.
Several years ago, I was at a customer's shop. An older lady had brought him her '76 Newport as her grandson had changed spark plug wires and it wasn't running right. WOW, was it NOT running right! It was completely stock, TQuad and computer and all, even low mileage and well taken care of.
Off idle, it was doggy, even at WOT, until it got enough rpm for the secondaries to open, then it was like you lit the afterburner. My customer was scratching his head on this deal as everything he checked was good. He wondered about the firing order, which was cast into the intake manifold runner. He called a few days later and said the grandson had crossed #5 and #7 plug wires, got them reversed. When that was fixed, the car ran perfectly. The B/RB Chrysler has the same firing order as a small block Chevy, where #5 fires first and #7 fires right after it does, in sequence. On a single-plane intake manifold, this consecutive firing can lead to #7 not getting a "full breath" as #5 and #7 runners feed from the same corner fo the intake manifold plenum, which was why the Holley Z-Line intakes were built (with a "resonating runner" connecting #7 and #8 intake runners, independently of the other runners, so #7 will get a full charge.
Glad the issue was reasonably simple and easy to fix!
CBODY67
