Well of course the rail broke! The m116 engines that have not had timing chains or guide rails ever replaced are notorious for doing this.

He's taking you for a ride. $4500 for ONE HEAD? Ridiculous. What's the extra $2000 for? BS. Rails aren't more than $50 in parts and no more extra labor if he's taking it apart to do the heads anyway. The bottom rails don't need to be touched until about 500k miles usually. The tops are the ones that get beat. Therefore, you don't need to pull the engine or take the front timing cover off or anything.