What model engine(s) do you have now? If fuel injected, will this be a bobtail swap with all the electronics included with the new engine? Many times the computer mapping is different between a 5.0 and a 5.7. If carbed, may need different jetting if just changing out the block& heads. Sometimes the drive would have a different ratio with a 5.0. In order to answer many of the questions you have we would require more details on what all you will be changing out.
Agreed, if its a carb engine, it is simpler, also it is possible for the ignition advance curves to be different. You have to do some research to get these questions answered, via looking in Volvo Manuals.
I just did this last year. Mine was carbed, and job went flawless. I purchased a new longblock and swaped everything over.
Listen to the group, good advice is pliceless.
