Shook my magic Fox Body 8 ball and it said more info needed.
Is the starter stock?
Are the starter and ground wires stock?
Is the starter solenoid on the inner fender stock?
Have you check the battery cables, starter cable, and ground cables for voltage drop?
Does the motor turn over by hand pretty easy with the plugs out?
You see where I’m going with this? Not trying to bust balls just the more info you can give us the better.