Most likely, the shim will not be needed. Since the gear-drive is a pre-engaged design, the distance to the flywheel is less critical. But if you want to, you can try to measure and compare the distance from the mounting surface to the ring gear teeth, and to the pinion teeth. The important bit is that they not drag when the pinion is not extended.
For wiring, I would definitely recommend keeping the original solenoid (or buying a replacement if you don't have an original). The solenoid on the starter both draws more current, and delivers more inductive kickback than the original solenoid; which will shorten the life of the (expensive) dash pushbutton. The pushbutton on the starter is also really handy when troubleshooting, adjusting valves, etc.
But since my original solenoid had already lost part of it's smoke, I wired so it only has to carry current for the new solenoid. I moved the existing starter wire to the other solenoid terminal (so it's hot all the time) and ran a new wire from the load side of the original solenoid to the terminal for the new solenoid.