You could use 2.4JTD head bolts. They are M12 and shorter than original. But you must use torque plate with this option as these bolts will make more distorsion to the cylinder than original ones.
Also don't tap the thread from start/top of the block. Leave it "blank" for first cc 10mm. This will reduce stress to the top of the block.
I was searching a lot for this subject, and ARP's M10 custom age bolts are not "stronger" than original M12 from 2.4JTD. Only advantage in ARP's are stud and nut sistem that is more reliable to tighten properly. But M12 ARP's will be bullet proof
ARP could make custom studs, no problem. But the price of these studts make them not profitable.