It sounds like you have a 160* thermostat. When it gets cold, a 160 won't cut it. 180 is better. I see you're in Texas, where you can get 4 seasons in a day. If you live up in the Panhandle, I'd recommend going with a 180 or 195 for the wintertime, and switching back to 160 in the summer if the vehicle overheats. If it doesn't, just run a 180 all the time. And if you can fix the "breathing" cab, do that, too.
I live in the Central Valley and it can get chilly, definitely not as cold where I grew up, at the base of Mount Whitney, and it gets down right cold there. I discovered at a young age that a 160 doesn't do well in the cold.
When I got my first '65 C20, it was being put together by grandpa, but he passed away before he could finish it for me. He had put a loose 3/4" piece of plywood over the SM420. Being 17 when I got it, I didn't know the danger of the u-joint being right there. But it was unbearably hot in the summer, and not driveable in the winter. Eventually I got a center hump, but it lacked the flange that goes between the hump and the firewall. I fashioned some cardboard to fill the gap because I had limited supplies and no sheet metal, nor the knowledge needed for working with sheet metal. That helped, but the cab still "Breathed", and it was a miserable "B" in cold weather. I eventually found a flange and that fixed the "Breathing", if you will.
My current '65, came with a breathing cab, and I have used heavy duty aluminum duct tape to seal the holes until I can get in there and cover them with sheet metal.
