First start question reguarding exhaust manifolds

We are putting together a 318 with a cam for my sons 1963 valiant.

Here's the question.

We are running the factory 64-66 273 exhaust manifold on the drivers side, and a 1967 a body 273/318 exhaust manifold on the passenger side.

We are going to have to start the engine and immediately break in the cam and set timing.

Can we do this with these manifolds open? I'd rather get it running before bringing it to an exhaust shop for exhaust.

Never fired a new (ish) engine through wide open manifolds before.

Thanks. Eric and Jason.