I don't have a '75 wiring diagram here, but:
The three fuses should be "MAIN" "HEAD" and "TAIL". Some fuseblocks hold 5 fuses but only use three, the other two are a spare for MAIN and a spare for HEAD or TAIL.
The MAIN fuse is likely 15A. The others are likely 7A. Using the 7A one for MAIN will probably blow it within a few seconds.
The fuses are not the normal AG3 type you find in automotive stores, they are a metric size that's very close to the AG3 but different enough that you should get the right size, an AG3 will work in a pinch but the contact area is a lot smaller and overheating and failure may happen in time.
The fuse block itself is occasionally a problem, the clips have wires soldered to them in the back and the soldering job is sometimes poor causing the wire to just fall off after a few dozen years.
The obvious point of failure when you lose all electrical power is the MAIN fuse. If you have a voltmeter (you should!) just measure from ground (frame or battery "-" post) to both ends of the fuse, you should have voltage even with the key off. If you do, try with the key on: you can have a very high resistance fuse or wiring fault that reads voltage with no load but shows up when you draw current.
The battery may even do this: I've had several batteries in cars and bikes that measure a healthy voltage with no load but won't supply more than a few mA.
