I added shore power to my 1978 S2 8.0B last fall.
I installed the power inlet/socket under the port lift-up cockpit seat, on the aft wall. If you crawl back in the aft berth, you will see a built up section in the fiberglass, where a wood block has been glued and covered with fiberglass (at least there was on mine). I have seen a "factory installed" shore power socket there on another boat in our marina.
The instructions for the socket said to drill a 2-7/8 hole, but I couldn't find a hole saw that size, so I used a rotazip bit. It was an adventure, but I eventually made the opening. Right after I cut the hole, I did find a hole saw bit the proper size!
I ran th 10 gauge marine grade wire around the back of the transom (in conduit) over to a breaker mounted near the bilge pump (gusher) located on the starboard cockpit seat. I mounted the breaker box to the wooden panel that separates the sail locker from the engine compartment.
I then ran the wire exiting the breaker along the top of the wooden panel (in conduit) to a ground-fault circuit interrupter (GFCI) outlet above the batteries.
I then installed an outlet on the cabin bulkhead above the ice chest and to the left of the sink.
You can find some photos of my shore power project at impliedconsent.us
If you have any questions, feel free to email me at the address listed on my web page.