You wouldn't want to connect the shower to the thermostat on the motor, that would drain all your coolant.
My best guess would be to connect the shower upstream of the external thermostat, so that you would still get hot water even when that thermostat is closed. In fact in the first pic it looks like there are a couple of plugs that would be ideal.
Edit: Sorry I didn't read the question properly. Heater not shower...
A heater would be fine to connect to the closed side (coolant for the motor) since it will be circulated.
Those hex head plugs in your block should be heater hose connections.
Last edited by jtnz; 05-18-2011 at 5:20 PM.
Reason: additional text