If the voltage during cranking at the ig sw is not within 1/2 volt of battery voltage, then some voltage is getting lost between them. The usual culprits, in order of probability are: the firewall connector, the fusible link, or the ammeter gauge.
The most voltage you can get at the switch is battery voltage.
So if it's less than 9.5 volts at the sw,(and therefore just a tic more than that at the battery), and the aforementioned connections are good, then the battery is is either in need of charging, or dead, or shot; or the starter is drawing way too much current, making the battery appear to be shot..
