If I can shed some light on this problem I've got a couple of 240 series Volvos one of which has had a succession of head gasket problems for the nearly 18 months. It's a 240 estate 1984 vintage but was bought with only 176,000 on the clock (which is still pretty modest for one these as I know someone with the identical model/year which had done 600,000 on the original motor!) We had the car into the garage with the typical "cappucino" goo under the filler cap and resgistering on the dipstick which always signifies big bucks at the beginning of 2008. Had the cylinder head machined, gasket replaced etc but a few months later same thing happened again! Needless to say the garage was a bit shamefaced, tossed the original head out and fitted another machined and crack-tested head just for the labour cost. Guess what? Another number of months later gasket goes again!?! This time around he secured another head and did the whole job AGAIN gratis. What he told me was that once a head gets to that sort of age it can happen very readily, he was pretty dark on his crack-testing cohort though! As far as the head gasket blowing he said that it was the coefficient of movement bewtween the cast iron block and the alloy head that caused the gasket to chafe and eventually let go. BTW I don't neccesarily believe a conversion to a 4 from a six cylinder would be that big a deal as long as all vital components are available for the swap (engine mounts will differ just for a start...)