The stock intake is actually quite fine. If a vacuum gauge is installed in the intake, very little pull is shown even at high revs there for it really not very restrictive at all. And it does pull cold fresh air from outside the engine bay on all vintage idi's. Im always used to think about ways to build a "ram air" setup for them but without a turbo, there is really no way to stuff more air into it then what it already gets. A stock intake with a quality well cared for filter is just fine for stockish engines. Just my $.02 worth.
I have had many idis of all makes, messed with the intake ducting on ALL OF THEM except my current fleet and I never once noticed a difference in performance in any way, just more noise in some cases, and always a rattier looking engine bay