The bolts are designed to stretch when correctly tightened, so reusing isn't a good idea. It's not a nice job and you don't want to have to do it twice. Also bear in mind there are three different thicknesses of head gasket, you need to fit the same as the one that comes off. Earlier versions have four individual head gaskets, later ones have a single steel gasket. If you suspect you have a blown head gasket due to coolant loss, be aware that it is very common for one or more of the cylinder heads to crack giving head gasket like symptoms, especially on the early diesels. These are identified by the glow plugs situated on the side of the heads rether than the top.