Given enough shoving and rotating you can get the thermostat housing and the pipe shoved in between the heads as an assembly. It does require care to avoid smearing the RTV you've laid down on the block but you do have enough room to lift the housing up and check that it's got enough to seal all the way around before bolting it down. And of course, make sure to line up the rear clamp before tightening the housing down.
If you do clip a bit of the RTV off, using the little plastic nozzle they provide and a small screw driver I've repaired it while lifting the thermo housing out of the way.
