|A short summary of what I did a few years ago: http://www.cc27association.com/f3/toast/toast.asp?sub=show&action=posts&fid=2&tid=8021
Note the reference to the flush-mount through-hulls. You'll most likely face the same thing.
Your best bet in my mind is to buy a $5 set of plastic calipers and measure the outside diameter of valve tailpieces and match-up the sizes that way. Who knows what's been done on your boat since it was built? Just be careful that nobody has upsized anything and used reducers. I, for example, increased the diameter of the engine cooling water intake (to what my fuzzy memory can't recall). The reason being if I re-powered in the future to a diesel I'd probably need it. There is validity in sealing off the head discharge and re-plumbing the holding tank (I assume you have one) to suit.
1979 C&C 27 MkIII, Hull No. 780
North Saanich, BC
[For what it's worth, when I re-powered with a Yanmar 2YM15 diesel, the mechanic said that no intake upsizing was required from original, and so it has proved (I can shut the seacock down to less than 50% flow before the pyrometer starts to complain, tho' this probably is not true for boats in the southern US, where people have reported summer water temperatures in the 90-degree F range). The exhaust line, on the other hand, had to be significantly enlarged. - Admin]