Hello
I just changed the head gasket on a 740turbo that I bought with a blown gasket. Dont forget to use new headbolts they are torque to yield and one use only, that is how I got my car for $200. A caution I turned the crank using the pulley, it is dampened and the mark can turn without turning the crank. It caused me a timing problem I could not figure out until I checked where the piston was in the plug hole, if you turn the crank get a socket that fits the bolt. This might seem like I am a bit lame but I dont think the rear seal will be much easier with the head off and I usually find it is better not to get any more complicated on a job than absolutely necessary-mind you there are people on this list who are more experienced with volvos than I. That said it is not a really difficult job on a 4cyl if you have everything you can probably finish in one long day.
Aaron