75 318 Smog Pump Removal

Should be fine. I assume you have an air pump that has the plumbing that goes to the exhaust manifolds - since the the thing as you note has a check valve - it will be fine.

FYI chrysler used to sell some heavy flat stamped pieces intended for closing off the air inlet on each exh manifold. I bought a pair years ago, but have not used them yet. Take a look in the back issues of Mopar action regarding a gran fury or diplomat they tweaked, the part numbers are in that article, probably 5-10 years ago.