I would almost say don't even try this job without the following:

-A good allen wrench (19mm) and a cheater bar (lenthen's the handle lenght of the allen wrench
-A reasonably hot torch
-Perhaps some ice?
-A BFH (Big f'in hammer)
-Perhaps some penetrating lube.

When you put the heater and adapter in, do us a favor and put a dab of anti-seeze on it