405: bleeding effectively

[Pug_XUD_KeenAmateur]

I'm gonna have to replace the thermostat (which is either absent or jammed open) on my 405 prior to the winter, which even in this hot weather generally runs at a little under 70c*

Despite the obvious simplicity of this job, I'm very loathe to touch the cooling system having had all sorts of head gasket nightmares on its predecessor (despite a replacement head gasket then a replacement engine)

Can anyone please give me advice on how to bleed the system effectively; there are a couple of difficulties with the XUD in its 405 application...
...the rad, which has side rather than top & bottom tanks; is not the highest point in the system and neither does it have an expansion tank
...the bleed valve in the relevant hose always seizes solid

Or failing that, any suggestions to blank off the rad, me thinks a sheet of something perforated within the subframe, in front of half the rad?

Thanks in anticipation of not freezing to death this winter !

Normally aspirated 1.9 Diesel by the way
*cooler since I rebuilt the front end following a prang, using a sub-frame from a non-aircon equipped vehicle and therefore couldn't put the air-con 'rad' back; thankfully the rad itself was undamaged.