Ford never factory installed exhaust manifold to cylinder head gaskets on any classic Cougar from 1967-1973. Most Felpro and other gasket kits include exhaust manifold gaskets in their complete engine gasket kits. Your manifolds should seal up fine if you clean the manifold surfaces with some sandpaper and use a thin layer of silicone sealant. I like using either black or blue colored silicon sealant, hi-temp sealer is unnecessary.
289/302 Engine Tip. 351 Windsor exhaust manifolds are a good upgrade from the restrictive log style 289/302 stock manifolds. Very similar in design to the Mustang K code HiPo exhaust manifolds.
*H code 351’s were delivered with both Windsor and Cleveland engines in 1970. Not sure what you have? Windsor engines had 6 valve cover bolts. Cleveland engines had 8 bolts holding down each valve cover.