Right tools for the job...

That is just a ½” Craftsman 3/8”dirive socket. It fits both upper and lower nuts securing the manifold nicely. This particular Husky brand socket set was purchased from a pawn shop in 1971 for 10 bucks to repair my first Dart. I cracked the Husky ½” socket in the early seventies when using a helper on the breaker bar, and replaced it with the current Craftsman 1/2" socket pictured.

Perhaps you have a set of overseas produced sockets that are a bit thicker due to lesser quality tool steel.