Reusing the old Stud Bolts is simply not worth the risk! The old items will have stretched and have possible thread damage. Either way they will be difficult to torque up properly!
Other lengths of M8 Stud Bolts with the same part code that you may wish to consider:
Use new nuts and washers as well. Interfacing worn parts with new is not such a good idea. Order all the gaskets, seals and other new fasteners at the same time and reduce the shipping costs with one larger order.