Normally, I hate fixes-in-a-bottle. However when faced with a $12k estimate, you have almost nothing to lose by trying. I know one pro shop has reported excellent results with Bar's Leak HG-1, they have fixed multiple vehicles with similar symptoms as yours. Money back guarantee if it doesn't work, and you are only out a couple hours of labor to try it. If you do try it, follow the instructions closely. At a minimum it should let you drive to the dealer:
https://www.amazon.com/Bars-HG-1-Blown-Gasket-Repair/dp/B003RGIWO0/
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If the head is slightly warped it may need to be machined flat. If it's badly warped, which isn't likely, it would need replacement.
ANY OHC engine, machining a "warped" head will allow a flat mating surface head to block, block has a less chance of warping vs a head.
So now you have two flat surfaces (head and block) for proper head gasket sealing but now have warped out of alignment cam journals being not dead nuts straight concentric aligned journals. Totally stupid waste of money and effort unless you want a near future kaboom!
Any OHV engine the rocker shafts can tolerate a little bit of "warped head" out of alignment problem with the hydraulic or hard cam lifters requiring valve adjustments back into spec.
Did any of these so called Rocket Scientist repair facilities ever perform a compression bleed down test to pinpoint which bank has the problem or problems of both banks after making a steam engine out of this vehicle? Anyone ever read their oil as well engine temp gauges while driving? Idiot light, how did it get that name?
Over a year ago I (we) picked up a pair of 5.0 litre heads for just $250 a pair before tax and deposit off a clean mechanically running low mileage LR that was smacked. Pulled the plugs looking even color, oil clean (what was left after wrecking yard drain), no sludge internally only slight varnishing a "go for it" removal and install on a friends LR4. I got roped into his bad vehicle purchase adventure with so far a success still running 20K miles later.
Blown head gasket with ant-freeze washing down the Cats destroys them cats not alone the O2 sensors, think about their added cost to your repair bill plus gaskets and head bolts but run only run ARP head bolts on my engines be it the old push rod D1 or drag race engines.
Engine swaps can be a good thing, last resort usually have a 30 day return exchange, labor's on you always.......~~=o&o>........