Turn signals not working, blowing fuse
Looks like I'm wrong about that. Earlier cars didn't have a fuse on them but the FSM says your '73 does.
If I'm reading right, that fuse is shared with the heater.
Since its failing as soon as you turn on the key, first check that both the heater fan and the turn signals are off.
If the fuse opens with both switches off, then probably one of the wires between that fuse and either switches is contacting ground.
EDIT
Looking at the wiring diagram, Fuse 2 should simply be A/C.
The fuse list from the shop manual then gets read like this.
So Turn and Stop are on the same fuse, number 3; along with the reverse and heater fan.
If I'm reading right, that fuse is shared with the heater.
Since its failing as soon as you turn on the key, first check that both the heater fan and the turn signals are off.
If the fuse opens with both switches off, then probably one of the wires between that fuse and either switches is contacting ground.
EDIT
Looking at the wiring diagram, Fuse 2 should simply be A/C.
The fuse list from the shop manual then gets read like this.
So Turn and Stop are on the same fuse, number 3; along with the reverse and heater fan.