small block chevy question

i have heard a little bit about using different "restrictor" or something things in the intake manifold gasket or somewhere... the instructions for installing a certain 350 crate engine say to make sure and use the right restrictor set. WHAT THE HELL IS THAT? lol. ive never heard of that before, which must just reflect my lack of hands on skill lol... can anyone tell me the purpose of the so-called restrictor? maybe a rev-limiter or emissions requirement?

a little update, i did some more research and found out it has to do with two heat riser passages in the block, one on the left and one on the right, sometimes one of them blocked, both, or none possibly. Still have no idea that they are for or why one would block one or both of them lol

There is a valve on the output of only the passenger side exhaust manifold, which closes when cold. While closed, the exhaust gasses from that side are sent thru a passage in the intake manifold and out into the exhaust manifold on the other side. This aids in cold engine performance and gives faster warm up. Once warm the valve will open and allow the gasses out as normal. When functioning properly on a cold morning, you will see the exhaust exiting the drivers side pipe only. (on a dual system) If you're running headers you want to block it off completely with the proper intake manifold gasket.

EDIT: Only the older models had the heat riser valve. (my 65 did) I believe the later years that did not have the valve still used the passage way, with restrictor, to aid in warm up.

ah ic ic thanks for the clarification...