Sounds like you have at least three different problems. Work on one circuit at a time. Wire a circuit and test it before moving on to the next one.
Do you have a volt ohm meter? If not, I would get one; they are not expensive. My VOM is one of my most frequently used tools.
I hope you have a voltage reducer for the gas guage. If not, it's probably fried. Assuming you have the reducer, with the ignition on, check the voltage at the guage - should have 6 Volts. Switch ignition off. Next, disconnect the sender wire at the tank end. Check the resistance between the sender and ground. The resistance should read between 0 an 30 ohms. (0 ohms when tank is empty, 30 ohms when full). If the sender reads 0 ohms and the tank is not empty, the sender is bad. If the sender is OK, replace the wire on the sender and disconnect the sender wire at the back of the guage. Check resistance between the sender wire and ground. The resistance should be the same as what you had at the sender.
Headlights. You say the high beams blew the fuse. Was it OK on low beams? Did you check your dimmer switch before you installed it? I once had a dimmer switch that shorted when you stepped on it. Pull the connectors (sockets) off the back of both headlights. The connector has three contacts. Clockwise, the contacts are high, low, ground and none (no contact opposite the low beam contact). Using your VOM, check for shorts between each of the three contacts and ground. One of the contacts should be ground. The other two should be open.
Parking lights. The turn signals work; did you convert the parking lights to use dual filament bulbs or are the signal lights separate? Do the tail lights and instrument lights work OK?
