Did all 1968 firebird 350's come with a heat riser valve on the manifold? My manifolds are stock, but I can't find it.

Will I get any benefit by changing my stock air cleaner with a flow model?

Open element aftermarket air cleaners are inexpensive, so give it a try. I do find that I get a little better performance and cooler running with the open element. Just be sure to adjust the stud to clear the hood.

They all must not have them. One of the Pontiac Tecnical Bulletins discussses the eliminination of them during the year on most V8s See: http://firstgenfirebird.org/dtb/docs/68-I-56.pdf