Is overspray on cylinder head ok?

That's a hell of a lot of work to go through AMC just to take a chance of blowing up the whole works.

Its very easy to just take the heads off now and remove the basepan while the engine is out and check everything to be sure the engine is actually rebuilt. A little elbow grease and a few gaskets is a small price to pay to be confident the engine is sound
What's life without risk? If this is a craigslist/Facebook engine that didn't cost a lot... if I'm staring down tearing an engine down either way.... 16 go, 17 stay...

Now, if OP has 10 grand into this, then absolutely, rip it apart. We all can agree this isn't good or the right way to do something. A grand? Send it and risk it and if it goes it had to be torn down and redone anyways. 20 minutes of run time to find out isn't going to grenade the thing.