Colin,
Do a search on Pelican and you'll find a lot. Basically, the '74-'77 2.7l engines had the following issues, mostly related to running too hot:
1. Heat exchangers that make the underside of the case too hot. This can cause differential expansion of heads/cylinders and head studs and sometimes leads to pulling of head studs. This can be fixed with dilivar head steads which expand at the appropriate rates. (Do a search for "2.7 and dilivar" and you'll find a lot of info).
2. 5-blade fan. Some say it doesn't flow enough volume of air to cool the engine. This can be replaced with an 11-blade fan.
3. Cam chain tensioners are mechanical and can fail. If you replace with hydraulic tensioners, the problem is fixed.
I've got a '77 2.7l 911S and love it. All the 2.7l issues have been addressed and mine runs like a rocket and I don't have any issues. There has been a recent thread regarding the 2.7l motors and how option has changed here:
http://forums.pelicanparts.com/porsche-911-technical-forum/827589-funny-how-nobody-trash-talks-2-7-anymore.html
- Mike