318 la motor

George Jets said

Just chop the factory air tube that goes into the manifold 3/8" long and weld that tube shut with a wire feed.

Then bolt that on with the factory bolt and the available gasket with your exhaust manifold gasket set.

Those Truck manifolds are available brand new aftermarket if you can't find good used ones. Also remove The heat riser butterfly and shaft and weld those steel sleeves shut too.

Click to expand...

Or you can just tap the air feed hole for (I believe) a 3/8 NPT pipe plug.
Actually, you don't "have to" do anything at all- the air injection passageway in the manifold is completely divorced from the exhaust, if left open they just go from "nowhere" to "nowhere". Provided the ports in the head, if they are there, have been tapped and plugged with allen screws.
Some of the manifolds may have an O2 sensor hole by the outlet that needs to get plugged.