Actually that is not head bolts stretching. The bolts get blamed for that but what happens is the washers settle into the head a small amount. If you inspect the head bolt washers they distort and due to a hot area at the rear of the head they have been prone to "loose torque". I find if you pull up the torque a few times early in an engines life it will live trouble free for a long time.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.