Looking for the ground for starter an tranny

The starter motor grounds through the engine block. Battery cables and connections all clean and good, positive and negative? You could use redundant ground for starter motor, see if it helps? You could use one lead of jumper cables, one clamp goes on starter motor body or mounting bolt, The other end goes to negative side of battery. Does it crank? You could take starter motor off, lay it on the ground, hook up jumper cables properly from starter motor to battery, positive and ground. At the least the starter motor should kick out drive gear and spin. It's a no load test. Just my opinion.

All you need is the bolts.