After 6 weeks and many small parts I have my 78 Spider running (a plugged fuel tank sending unit). Now it immediately overheated and the fan wasn't running. Thermostat? Electrical? What's next?

Thermostat is stuck or you have a air bubble in the system preventing the thermostat from opening.

1. Put the thermostat in a pot of boiling water to see if it opens, and use a cooking thermometer to make sure it opens before 200 degrees.

2. If the thermostat is working you need to better burp your coolant system.
2a. Make sure your heater is in the open position
2b. Fill the car on a massive incline of 40 degrees or more. If you can simply by a Burp Kit/Flush Kit so you can fill the coolant system from the highest point in the car. That being the long rubber heater hose (the one that goes over your exhaust cam).
2c. Squeeze the heck out of your lower radiator hose to thermostat while you are "burping" your car. This way you pass all air bubbles away from your thermostat. A simple trick many people do is to drill an 1/8" hole in the thermostat so air can "burp" itself.

Also, if you do the burping method with the Burp Kit you don't have to incline the front of the car.

To be honest I got most of that out of the Vick's Catalog. They did a pretty good right-up on coolant burping.

Monkey Out!

Another trick is to drill a small hole in the thermostat's plate that opens to allow any trapped air to pass thus preventing the problem. Still have to burp the system as best you can. Test the fan by grounding the black wire. if it comes on you are good. The temp sensor can also go bad.