The 300 gas engine tractors that I have seen the solenoid is mounted on a bracket with a cable from the solenoid to the starter post as mine is and in the lower 6v pos gnd pic.
The the diesel was 12v pos gnd with the solenoid mounted on a bracket attached to the starter post using the bracket as a conductor.
The second pic is a 400B 12v pos gnd same as 300B gas engine. I think the wiring is the same for the 300, 300B, 400B gas except lights not sure though.
I converted this 310 & a 311B to 10si 12v neg gnd 3 wire alternator, 4 terminal starter switch, 12v gas & temp gauges w/senders, lights, coil and generic solenoid. I didn"t pay much attention to the original wiring as both were a rats nest, not complete or functioning except to start/run the engine directly off the battery so mine are not wired as original.