Check voltages from the battery to the back of the fuse block, then check again on the front of the fuse block. From the fuse block check to the back of the ignition switch, and from the switch to the terminal on the solenoid. If the battery has only five volts it is dead.
There are fusible links in the primary wiring that could have blown, there could be corrosion where the fuse block bolts together causing excessive resistance, and there are fusilble links as well as fusses in the secondary wiring.
The factory wiring was inadequate for the loads in the car when it was first built. GM is famous for being cheap and copper has always been an expensive metal. It has gotten even more expensive today as we have depleted all of the readily accessible copper ore on the planet.
Copper corrodes (turns green) with age which causes resistance in wiring terminals. Additionally it easily work hardens, so vibrations cause the wire strands to crack, which results in fewer and fewer strands of copper trying to support the electrical load (which were too few to start with). This reduced wire gage can cause an electrical fire as the load heats the few remaining strands to incandescence.
I have no interest in restoring a car to factory specs; I would rather enjoy my cars so I yank out the factory wiring and replace it with a 12 fuse Painless 14 gage (or heavier gage depending upon the circuit) wire set along with a Modern CS (one wire) alternator rated at 160 to 200 Amps to handle any load that I might have.
By doing this I not only maintain a constant voltage at all times and engine speeds with the one wire alternator that powers the car, but I have nearly doubled the size of the wiring to handle the electrical loads and reduce voltage drop per linear foot with the heavier gage wires.
Your SI style alternator was rated at only 36, 45, or 63 Amps max at 3,600 RPM and was nothing more than an on board battery charger; with all electrical loads being running off the battery draining it faster than it could be recharged. this is why your lights dim at a stop sign and your wipers stop working at night in the rain. (I need more power Mr. Scott!)