Is it still trying to start? You can remove a plug and with the plug wire on the plug and the plug body grounded, see if you get a blue spark at the plug electrode.
The HEI distributor's coil needs to get full battery voltage. The '69 truck originally used a points-type ignition system that used a resistor wire to the coil positive terminal. If this same wire is now being used to supply current to the HEI coil, it will not have the voltage it needs to work right.
So use your voltmeter and w/the coil "+" wire removed, and the key "ON", note the voltage- it needs to be 12 VDC- plus.