My suggestion is to assess the conditions, then replace as needed.
Engine compartment usually has the most damage from heat, atmosphere, and mechanics doing work.
Assuming this is a US export, LHD. M&H probably makes the best. Regardless, save the original for reference and parts in the future.
*Engine harness for '68 big block.
*Battery cable harness which also includes starter solenoid to starter wire. (Not sure M&H makes that one, but others do. If using a ministarter, some of the replicas are too short reach without strain.
*Transmission on a '68 is probably just the the neutral safety wire, although depending on when it was built it might have the reverse switch wire as well. Very imprtant to determine which arrangement your car has or will have.
*Battery Ground (earth) is seperate cable. Using one with a body ground will not hurt and can help.
* Engine to body ground.
* Instrument Harness. This is the critical one on the inside. It includes the main feeds.
There is no reason to get into converting the ammeter to a voltmeter unless you are adding electric loads (EFI, electrc fans, etc). Alteration is not plug and play.
* Console, if your car has that option.
* Body - this connects at the driver's footwell and goes to the interior lamps, tail lights, fuel tank sender, and reverse lights.
MSD 6A or 6T. You will need to make up a wiring connections.
Distributor. Points distributor is easier - connector to mate the white MSD wire.
Magnetic pickup - Must properly get the pair of wires connected with the correct polarity.
On/Off signal (smaller diameter red wire on MSD) Use a piggy back to connect at the ballast resistor or bolt to the coil feed wire's ring terminal after removing from coil.
Regardless, the original coil positive wire must be removed from the coil, secured and insulated from ground.
Run the MSD coils wires to the MSD.
If you want to get fancy, you can run the MSD main power (heavy red) direct from the alternator. Otherwise you can connect it to the battery at the starter relay. The load is small enough that its not major concern.
Headlights. With a decent E-code H4 (Cibie, Bosche, etc) and lamps that will draw 9-10 amps, add a relay harness. Or, if using LED, then that's probably not neccessary.
