The only Pontiac I've played with the RPM intake on was 455's. Had one on the dyno and played with spacers. It liked a 1" open the best but wasn't huge gains. If I remember right it picked up 7 or 8 hp. The 4 hole spacer we tried lost power, just didn't like it for what ever reason. I think one possible reason is because the Pontiac version of these RPM intakes don't have the divider cut down like other engine brands do with the same intake.
Either way, it may not apply, not sure how a 400 might react differently. Easy to swap around though and experiment if you have the time.
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