Sam, while I have to agree with Mr. Creech because he's one of the resident Pontiac experts, I am of the opinion that you have installed the correct manifold . The RPM will make more power from 1500 to 6000 than any other manifold you could bolt on the motor, in my experience and opinion.
And just for grins and giggles, let me ask you.....when is the last time you needed killer power under 1500 rpm's??????????
Tell these "few different people" to get their sheets together or shut the hell up.