There is not a lot of hood clearance once the intake, carb, and air cleaner are installed. I'd suggest mocking them up in car, sealing the air horn with duct tape or a coffee can lid and putting a big ball of clay on the top of the air cleaner stud. Lower the hood by degrees, checking for crush on the clay ball. Otherwise you may end up with the reverse hood dimple so often seen.
You can fit a Performer RPM under the hood if you use a drop base air cleaner, which may negate some of the gains of having the taller intake. A '68 I worked on was mated to a Holley with a 3" air cleaner, and it contacted the hood. I moved the intake to another '68 I worked on, mated it with an Edelbrock carb and drop base, and it fit.
The '69 Pontiac cast iron intake flows just as well and would perform and fit beautifully on your engine with a stock height/drop air cleaner. I usually use the stock air cleaner, but with a smaller chrome lid that leaves the outside of the filter exposed. For show, I put the stock chrome lid back on. That way, there is no snorkel restriction and a whole lot of filter exposed.
1969 Goldenrod Yellow / black 400 convertible numbers matching
I wouldnít use anything other than a factory intake. They flow as good, if not better than aftermarket ones. They are good looking pieces, especially when they still have cast flash that will cause lacerations requiring sutures.
If thatís not a ringer, you can buy them all day long, sometimes for one-tenth of the cost of an aftermarket. Weight? If thatís a concern, you can gain tenfold weight transfer by installing the battery in the trunk.
As for factory breathers, this on isnít correct, but it has a very natural looking on the car, lots better than hokeyass-looking chrome under the hood. The vacuum hose off the hot air system is a dummy.
Itís off a í78 Grand Prix 400. Breathers during this era were engineered specifically so that you could flip the lid and have an open element, a violation of federal emission laws since, I think, 1968. There are only a few years of which you can flip the lid. Once the feds caught on, they passed a law whereby an automotive manufacturer cannot engineer emission components (closed element breather is considered emission equipment) that can be assembled incorrectly.
They appear to be just about the same height with a slight edge going to the 2156. I have not used either one so I have no direct history.
400, bored .040, standard pistons/rods, semi-mild Comp Cams hydraulic cam (XE 51-222-4) rated at 1300-5500 rpm. Stock 62 heads looking at just under 9.5:1 if my math is good.
If I recall, the taller the manifold, the better low-end response? Jeez, I just don't remember. I have a couple of weeks yet to decide as short-block is nearly assembled.
Anyway, thoughts or direct experience on these manifolds would be much appreciated.
I've not tested a bunch of manifolds personally but alot of reading from others who have, I would try not use either of those you have given other options. As mentioned the stock manifold is a fantastic piece and able to support a wide variety of of builds from mild to wild. The performer RPM is a good manifold if you have to have aluminum. And to be clear it's the taller runners/intakes that support the UPPER rpms.
It was brought to my attention from a fellow FGFer with a side by side comparison that the factory intake has larger runners than my edelbrock mfld. I never noticed this until they were side by side. The factory manifold was noticably larger in runner width and volume.
I have a 69 bird and with the edelbrock mfld and a 1 inch spacer with a Holley carb, it fit under the hood. Not sure what the stock mfld or my quadrajet will do for height, so I'll have to check when I get to that point.
I have a P4B, 1" 4-hole spacer, Holley carb, and Summit air cleaner. No issues.
I don't know how much drop the summit air cleaner provides, but it's not much. The PB4 is lower than an RPM which people have been known to install. I think the Holley may not be as tall as a Q-jet, not sure. But again I've added a spacer and everything still fits.
One thing to check is the body mounts. Really old mounts will allow the body to drop relative to the frame reducing hood clearance. It's really good to check these anyway, since it can affect body lean, reduce structural integrity, and in an accident allow the frame to either give away from the body, or allow the frame to shift rearward. Since the steering is mounted to the frame, guess what's going to be coming at at you if that happens. Hint, in a frontal collision your face will be traveling towards it, while it travels towards you.
Someone on the PY board was going to do back to back testing with the P4B against other intakes, but it hasn't happened yet.
Either one should work with your combination. You could try each one and see what works best. One trick on Pontiacs to seperate the crossover from the rest of the intake. Then you can swap intakes without draining the coolant.