Your 3 post starter solenoid will work fine. A terminal block is just a place to hook wires together. The amp gauge or a bolt and nut wraped with tape will do.
Your old regulator and its wires go. Your alternator has a regulator built into it.
For a 800 with the 3 wire alternator you will need a one way diode from radio shack for the wiring.
You will also need a 12v to 6v resistor for the wire that goes to your old 6v coil. Most just replace the coil with a 12v version Napa part #IC14SB for less than $20 and then you do not need the resistor.
Here is a wiring diagram for you. Feel free to print it out and use it to help you with your tractor.