Is it worth to fix?

To replace the head gasket is about 10 hours work, with over 300,000 miles on it it probably could do with replacing the piston rings which would double the amount of time to do the job and then if the head needs to be milled, or a valve job needs to be done things can get very labor intensive. They were a great truck, a mechanic going into the project knowing the truck will need a lot of labor to fix it may make a good offer on your Land Cruiser. I have seen higher mileage on these engines and the trucks just keep going.