Replaced starter on JD 318. Before I hook back up, which post is positive terminal? I feel that it is lower one, because upper one has a jumper wire from solenoid to starter.

Hi,
Have reason to believe the one you need is this on:
Our John Deere 318 Site
And your starter is the one in the dashed box. Battery attaches to top post(the one going to the shift solenoid) so that it can shift the pinion into the flywheel. The other is the ground. Best to jumper it(ground) to the frame and test before putting the engine back into the frame.
A second schematic is also at this site. It is:
Our John Deere 318 Site
Notice the serial number ranges and grounds.
Both have an option beneath them to get a pdf version for Acrobat Reader(recommended for printing.)
Peace,
Clarke