Blown Fuse #11

You are probably correct that your fans have a common fuse. It would be worthwhile to rewire the relays to have separate fused path. Pull power directly from the battery with inline fuses and avoid the ancient fuse panel altogether.

Check that your fans are free spinning and not hanging up on something, or have internal friction. Also make sure your alternator & battery is tip top and is putting out 12+ volts. When voltage drops, then the current a device draws increases, and could max out the fuse.

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Could it be that my 72 has the connection similar to the one in this diagram, where the fans power ran off the same pink wire(below) that's fed by fuse #11.

Yes, it's possible. The early cars had the fans wired to one fuse. Somewhere in time, a change was made so that one fan was on fuse #11, and the other fan was connected to fuse #12. The wiring diagram that I have shows two wires leaving one fuse, with one wire (black) going to one fan relay, and the other wire (yellow) going to the other fan relay. You might get lucky and have two wires going to fuse #11. If so, that makes it easier to add another fuse to one relay.

If the wiring or fans haven't been changed by a previous owner, then something has increased the current (amp) draw, as the fans originally worked when connected to the one fuse, although it was marginal.

If you do as Dave suggested, that would be the best way to eliminate some voltage drop. If you do source power directly from the battery, and you have an ammeter in the center console, whenever the fans come on, the ammeter will show a charge. This does not hurt anything. While possibly confusing at first, this indication of a charge (when the fan or fans turn on) shows you the extra current (amps) that the alternator is producing to run the fans. When the fan(s) turns off, the ammeter needle will return to zero (unless the battery needs charging).

John