You're going to need a Ford-type starter solenoid-just about any one will work, up to about 1996-or an AMC/Jeep solenoid, or an AC Delco unit (p/n U939), along with two new cables. Mount the new solenoid on the firewall or the inner fender panel, making sure the mounting surface provides for a ground.
With a strip of heavy copper, make a lug that connects the battery cable terminal on the starter to the "S" terminal on the starter solenoid.
Run a length of (minimum size #2) cable from one side of the firewall-mounted solenoid's large terminal to the battery's positive post.
Run a length of (minimum size #2) cable from the opposite side of the firewall-mounted solenoid's large terminal down to the starter solenoid battery connection terminal.
Move the wire that went to the "S" terminal on the starter up to the one of the small terminals on the new firewall-mounted solenoid. It may or may not be marked with an "S." You can shorten this wire if you like. This wire comes from the ignition switch (and perhaps the neutral safety switch) and is what activates the solenoid when the key is turned to Start.
Move the wire (if applicable) that went to the "R" terminal on the starter solenoid up to the other small terminal on the new firewall-mounted solenoid. Again, you can shorten this wire if you like, or you can actually leave this wire in its original location. When applicable, this wire goes to the coil, bypassing the inline resistor or resistor wire, to provide a full 12 volts to the coil for better initial start-up.
Don't worry about it if you have the wires hooked up wrong if the solenoid terminals are not marked-you'll find out soon enough if it doesn't work. Different models of remote solenoids may require you to reverse the small terminal wires. If not marked with a "S," you can easily determine which one is the start terminal by using a short jumper wire from the battery cable connection to one of the small terminals. Do this before you connect the cable from the firewall-mounted solenoid to the solenoid on the starter. Contact with the start terminal will cause the firewall-mounted solenoid to click, indicating that contact is being made.
An added advantage to using the firewall-mounted solenoid is that the battery cable connection on the solenoid makes a good location to get power for any additional accessories that you may want to mount, but remember to use a fuse in the system.
I should mention one more thing before leaving the subject of starters. You just don't bolt in a GM starter, and the same goes for those on AMC (and most other) engines. The spacing of the meshing gears has to be adjusted properly to ensure long life, both of the flywheel gear and the starter gear. You do this with shims between the starter and its mounting pad. These shims are readily available at most auto-parts stores. The first adjustment is that the end of the pinion should be about 0.060 to 0.140 inch away from the flywheel when not engaged. This can be checked with a feeler gauge. The next measurement should be the backlash between the gears when they mesh. The easiest way is to measure the clearance between the top of the pinion gear to the valley between two of the flywheel teeth. The clearance should be somewhere between 0.035 and 0.060 inch to provide the proper backlash. It's impossible to use a standard feeler gauge in this tight area, so the trick here is to use a couple of paper clips. Yep, paper clips. The standard small ones will run between 0.030 and 0.034 inch in diameter, and the bigger ones are about 0.045 inch, so if you can get the small one in and the bigger one is a tight fit, you're pretty darn close. Again, the clearance is adjusted by using the proper shims.
You will need to use some type of a prybar to force the pinion gear into meshing with the ring gear to do your measuring. I have never tried it, but I've heard of people removing the starter cable and running a small jumper wire from the battery to the solenoid "S" terminal so only the solenoid pushes the gear to engage and the motor actually doesn't turn over.
One more thing: Use the proper bolts-the ones with the knurled section between the head and the threads. This provides a tight fit for the bolts and keeps the starter from moving around.