I believe that your only fuseable link in this car is near your battery.
This is where it is on my 66 Elky.
this particular link will protect (in theory) the whole car.
(I am assuming that you have a relatively unbutchered wire harness)
There may be another near the horn or alt. regulator. (my harness has been modified by previous owner)
Factory (sb the same as my 66) will have a wire from the battery to the junction block, which may be very close to the battery on your car. It is just above the batt. on mine, mounted to the core support.
14 ga black---goes to the firewall disconnect--(looks to be cigar lighter and related)
12 ga red----to horn relay
10ga red---horn relay>>alt>>voltage regulator
(From my 66 Chevelle manual)
If you were to connect the coil of the relay AND the feed side of the relay (pos 30) to this battery junction, you will get the results you want.
this is pretty much how I wired my 79 Chev C-10 about 15yrs ago. The fans on that truck will run for about 5 min before shutting off.
However--keep in mind, that if your fan switch is in the motor and not in the radiator, the fans will run a lot longer.
This is because you are only cooling the radiator and not the engine, where it will remain hot for a long period of time.
'Cause actually---the engine will get hotter after you shut it off. This will kick in the fans up to 10 min after you have walked away from the car.
Even with my sensor in the radiator on the 79, the fans will come on long after I sut the car off.
In the 59 however, I opted to use an ACCY terminal so that the fans cut off with the car. This thing (454 HO GMPP/Tremec 5-spd) just runs hot. I just flat quit worrying about it til it gets to 235. Sometimes the fans do not kick in till then. (supposed to come on @ 218 off @ 205)
I can shut this car down with a temp of 190, and 5min later be up to 230 when I start it back up. But it does cool down pretty quick. Runs about 200 all the time.