Thanks for the replies -- I will check the valve clearances, heights and cam lobes, plus get some auto glass specific sealant for the windscreen trim when I put it back.

Alschnertz's observation that it's not a North American car is correct: I'm in the UK. UK roads are salted in winter, and there aren't too many cars here that aren't rusted out by age 37. This Volvo has had an after-market underseal, and the underneath is still very solid. What probably really saved it is that it was exported to Australia in 1985 (there's a Queensland state auto compliancing plate in the engine compartment). It was re-registered in the UK in 2006 I believe, when the owner returned to the UK (same owner from new to 2007).