Your kit comes with a thermostat housing facing straight up, but I need one 45' tilt, what will work?

Any Chevy o-ring thermostat housing in aluminum or chrome in 45' or 90' will fit as they have the same bolt spacing as Chrysler.

Are the o-rings hard to find if I need them?

They are standard o-rings found at most auto parts and industrial supply stores (in Canada). The larger one* for the adapter plate is Dynaline # 54229 and the smaller one for the housing is Dynaline # 54226

*Outside Canada they are sold as #229 o-ring by McMaster Carr. Please note these o-rings are the same we use, especially selected for softness for this adapter.