Time to bump this thread with new info.
If you install aftermarket block heater, they don't mount to the rear of the engine. They go on the front. There are pros & cons to both types, installing it on the front is easier, but there are no threads in the freeze plugs there, so you rely only on a rubber o-ring to contain your coolant.
Here's how I mounted my Defa 230V block heater..
More pics at buildthread:
http://www.sr20-forum.com/members-rides/54209-dala-s-nx2000-56k-heavy-6.html