72 Slant Six running too cold

Dam...Wish I had Your problem. lol

Actually,I would stick with the 180 stat.

And yes different senders will work differently. Found this out by installing a Auto Meter gauge in my /6 car. Left the stock sender in,because it was fairly new, and it didnt read right. Installed the Auto Meter sender,much better.

Also,did you burp the system,to remove all the air in the system ?
Start the car,and let it warm up with the radiator cap off. Let it flow untill the stat opens,and burps any air out.

I wouldnt rely on the stock gauge at this point. Try a aftermarket gauge to get a good reading.