Engine Temp Question

[zy5 guy]

So I have a question about the temperature gauge. Temp gauge never worked until I realized it wasn't hooked up. Found the wire for it and hooked it up the other day. After driving the truck around for awhile the needle would only go up a quarter of the way on the gauge. Truck by this point should've been up to operating temperature. My question is, what does your gauge normally show for a warm engine? If it is higher than what mine is showing, what else should I look at? Could the sending unit need to be replaced, or the t-stat? I do have plenty of heat when I turn the heater on. Not sure what degree t-stat I've got. Here's a picture showing just how far up the needle made it. Truck is a '77 with a 305. Thanks! -Evan