It's still awfully cold

[Libby]

I posted a question a couple of weeks ago about my 67's cold air problem (lots of air, but no heat). I've since had the heater core worked on--apparently, it was stuffed full of crud. I also had a new plenum installed; I guess the original cardboard just wasn't meant to last 38 years . Anyway, I've got great air, but still little heat. The thermostat is a 160*, but if I use a 180* she'll overheat. Since I had this work done, I've noticed the temp. gauge isn't moving over quite as much as it used to, either. While browsing thru a Paddock catalog, I came across something called a "temperature sending unit". Now, I'm not real sure just what that is or what it does, but I'm wondering if it could be the problem. (is it painfully obvious that I don't know a heck of lot about cars?) Anybody have any ideas? I'm slowly driving my mechanic insane with this problem. Thanks for the help. Libby