4kq running cold

[Steve]

We just got our first blast of winter here and a problem with my 4kq has
resurfaced.  In very cold weather and at highway speeds, the car runs
cold (only 1 or 2 notches up on the temp gauge), sometimes so cold that
it produces almost no heat in the passenger cabin.  I had the same
problem last winter, but the weather got warm and I "forgot" about it.
Now, I'm traveling across PA for T-giving this week and I'd rather not
freeze my n*ts off driving through the mountains.  Has anybody else
experienced this and how can I solve it?  Put a piece of cardboard
behind the grille to block the cold air?  Replace the thermostat?  TIA.

Steve
85 4ksq