You already have the carb keep it, it's well suited for your 400. The performer series manifold is made for running from idle to 5500 rpm. If you want to race at 6-7000 rpm use a race manifold. From what I've read the 74 intake was not as performance built as the 60s manifolds, you may find some improvment over your cast iron unit using a performer but may not notice it on the street.
Jim Hand, who knows more about Pontiacs than I ever will, has done extensive testing and found no inprovments in E.T. or speed on the 1/4 mile track using the Performer over the iron intake. But he drives a full size station wagon, results could be different with a lighter car. He does use an Edelbrock intake on his car, so do I. I got 365 RWHP using an Edlebrock carb and Performer intake on a mild build 428. I used a 3/4" aluminium spacer. Since then I've rebuilt and use a performer rpm intake. I had to lose the spacer and get a drop base intake to fit under the hood with the rpm as it is taller.
If you want to change the intake you have to the performer because you think it will give you great increase in performance, don't. It wont. If you want to change it for some other reason go ahead it will work.
The performer isn't just a big square hole, it is divided just like the Q. Jet stock intake. There are two rectangular holes left and right just like the q. Jet right under the top plate of the intake. The holes in the stock top plate match the bores in the carb but it is open under, just like the performer.
If you get ahold of Jim's book 'How to Build Max-Performance Pontiac V-8s' check out the manifold section and see some of the modified intakes he's made.
