I don't know for sure the model years affected...but there was a recall on the gaskets used by Volvo between the manifold and the riser...if these gaskets fail it's possible for the engine to suck the cooling water for the manifold into the cylinders...I'd ask about that...see which engine years are affected...there was a thread about this over at
http://www.thehulltruth.com some time back..
My old 88 OMC 4.3 has an unusual design for the manifolds...the manifold and riser are cast as all one piece (called the 'batwing' because they kind of look like that) and I like this design because there is no joint between the manifold and riser to leak...unfortunatley they are hard to find but I bought an extra set when I saw the price of them starting to go up. Merc tried a similar design but discontinued it...don't know why...I'm sure it's more difficult to cast than a 2 piece riser/manifold....Merc now has their dry joint system...which makes a lot more sense than the old system where your engine depends on two little gaskets to keep the water out of it.
Keep in mind if you are in a salt water area...and this is a raw water cooled engine...you're only going to get roughy 5 seasons on the manifolds before you have to pull of the risers and inspect how rusted the sealing surfaces for the gaskets are...