Hi. Sorry to hear about your misfortune...but you really had to tighten down that nuts to shear it like that!
You should know, however, that you hardly have to tighten those thermostat housing nuts at all! The housing is sealed not by metal-to-metal compression of a thin "paper" gasket (as it used to be the case with american cars), but by a supple rubber gasket around the rim of the thermostat. I've never tightened those nuts more than "two fingered" application on my ratchet handle. Just snug -- and I've never had a leak. And that's about thirty years of doing this on Volvos, from my first one, a 164, through about seven 240's.