1976 Dart Overheating and other issues

Yuck! That t-stat housing says a lot!

Imagine that through the whole cooling system including the passages in the block and head. Without getting too much into it, I'd drain and flush then drive for a few days with some radiator flush.
Then, your rad is definitely restricted and needs to be rebuilt or replaced.

New water pump, hoses (formed, not those cheesy convoluted universal ones) 50/50 mix of green Prestone (unless you go with a new aluminum radiator) and you'll be good for a long time.