Feasibility of replacement of Subaru Outback 2007 3.0L Cylinder heads

It's a reasonable job once these wide engines are removed from the vehicle. I was quoted US$2-3K at a reputable Subaru workshop. As requested here's some docs to estimate doing the work yourself;

Subaru Legacy/Outback Factory Service Manuals

Subaru H6 Technician Reference Booklet

There's also a couple videos on YouTube and good resources at subaruoutback.org, note that some folk prefer to buy and install a used JDM engine over rebuild.

Your big saving will be in labour, but you're still going to be in it for over $1,000.

How many miles on the engine? How bad was it overheating? If it was at the end of one journey, and you then stopped and didn't drive it to overheating again, you won't need new heads (you can measure the deflection to see if a skim is needed)

The engine is connected to the gearbox by less than 10 bolts (plus the torque converter bolts). It's a tight fit, but if you have experience you can get it out and onto a stand in an afternoon (less, if you can get the car into the air to get the lower bolts)

Once it's out, there's a shopping list of things that it's best to replace, e.g tensioners, water pump, thermostat (with a Subaru one this time :) ), timing chains, plus a few other things. Note that if you put a JDM (Japanese Domestic Market) engine in anyway, you'd still have to replace all these.

There are a couple of Subaru forums around with good videos and text walk throughs - people are glad to help. Get hold of the correct Workshop manual for all the torque settings.

Once you've done it once, you'll end up taking the engine out for basic maintenance every year or so because it's so easy, rather than fighting to get those spark plugs in!