Reality is you will not know anything until you pull the head and send it to a COMPETENT machine shop. That head can be warped up to 1mm (.040") at which point you can machine it .5mm(.020) and it will only be warped .5mm. If it is warped .5mm or less then machining it is not necessary and you can run it the way it is. The valves are stellite coated and machining them is not an option according to Volvo. On turbo cars that needed a valve job we always just replaced them.

One good indicator of an excessively overheated motor is to look right down on the top of the block underneath the thermostat housing. What color is it, is it still red, is it kind of a burnt orange to brown color, or is it black. Obviously as it progresses from red to black it goes from bad to worse. If yours is still red then you will probably be ok, if it went all the way to black then you should expect to see the pistons melted in their bores and serious scoring on the cylinder walls.

Oh and to answer your question, I do not believe that the two cylinder heads are interchangeable but I have never actually tried to see if one would fit on the other.

Good Luck,
Mark