[Zable49]

I would check your fuseable link (+ cable from post to fender)... I would also check your main harness at the firewall. I've had the firewall block curoded causing the starter to not get signal to turn over. Then if thats not it. I know you said you checked you grounds, but I would make sure its grounded at these locations with no grease and paint in between your wire and item-
Battery to motor
Motor to body
Body to frame

Good luck, let us know what ya find.