Sounds like you are right to be wary .... maybe they quoted for a V6 by mistake? But he did mention loosening the timing belt so probably not.
I would call again, try to talk to a different rep and get the quote again without telling them you already have one. If they insist they can do it for that price, drop it off and get a written estimate. At least in MD, they cannot then charge you for work exceeding the written estimate unless they get your approval first. You live in a pretty lib state, right, so they should have similar consumer protections