[Mrmerlin]

Is this a 5 speed or auto?
If a 5 speed is the clutch disengaged fully are the TT bearings going bad?
Have you tried to turn the driveshaft with the clutch pushed in?
do this in gear see if the clutch and drive line are free
If an auto has the flex plate check been done?

Have you check the connections at the 14 pin connector above the hot post?
Look for corroded wires, Is the ground wire clean and tight at the engine block and chassis as well as both of the battery terminals clean?
did you try adding a jumper cable to the negative terminal and then to one of the shock mount bolts ,
could be a bad ground cable from the battery.

Are the big red wires that connect to the alternator and the starter and the hot post all clean and tight