Old style non-permanent anti-freeze, as in back in the sixties, has one temp rating, new permanant type uses 10 deg/F HIGHER temps.
So....a 170 degree 'stat of say...1960, would use a 180 deg/F 'stat today.
Also, you want a Robertshaw pressure balanced theromstat, NOT a flat plate style Stant. Robertshaw is also sold as Mr. Gasket, from speed shops. Flat plate type 'stats can close at higher rpms from higher water volume flow pushing against the flat plate restriction. P/B ones use a barrel design, bypass around their closing plug, and don't have the water closing problems/issues the flat plate types do.