Yes you can take the thermostat housing off as one piece. But i'm sure that you don't need to take it off.
Cheers Damien"SL/ENUT" Smith, The SL/E Fanatic!
A lucky owner of 2 SL/E Commodores, a rare VB SL/E and a 2 tone VC SL/E. Just need a VH SL/E and have the set!
If needed, take the housing off, but replace the gasket before refitting. Get some Permatex RTV 'blue' silicon gasket maker and lather it both sides of the gasket before refitting. It can be picked up at most auto joints.
Grind a small amount off the back of the upper and lower thermostat housing where the thermostat flange is, that will allow you to get the rocker cover on and off (with a bit of manuevering) in future. make sure you use the little rectangular spacers under the rocker cover bolts on tin ones, they help disperse the load a little more evenly on the rocker cover.