SoCalDesertRider wrote:
You have only top end damage at most (heads). You do not need a whole new engine. Do a compression test on all 8 cylinders and compare the results. While spark plugs are out, check for water in cylinders. This repair shouldn't cost more than about $3000, if it needs head gaskets and heads milled. If a head is cracked, it may be able to be welded. If not, a used head isn't too much money.
That shop that tried to roll you for $21,000 should lose their business license!
Before doing even THAT, I would start with the simplest fix: retorque the heads. If it cracked a head, this will not work. If it just lifted a gasket, it MIGHT work...but if not, it takes an hour or so and costs nothing more than, at most, a set of valve cover gaskets.