You asked for it!
Since you're in So Cal, and I'd assume you'd like to retain your A/C, things get a little trickier than your normal PITA heater core swap.
You're going to need the services of a a/c shop right off the bat to de-charge your a/c and capture your old R-12 Freon if you still have it. R-12 is the stuff that supposedly depletes the Earth atmosphere's Ozone Layer and should not be simply vented into the air. It has been banned for years here in the US and was last used in cars in 1989.
Since R-12 is 'bad' and therefore unavailable at any price in CA, and very expensive ($25/lb) anywhere else in the US, you should look into converting your a/c system to the more environmentally friendly (and much cheaper) substitutes like R-134a. You will have to change out a couple of your a/c components to do so (dryer, charge fittings, etc. ), thereby adding to your heater core swap woes. Sorry to be the bearer of bad news!
Now, as for the heater core swap itself, let's just say that I wouldn't want to do it again, and mine is the 'easier' 71-73 model! I went so far as to do a 'delete a/c' on AlphaCat because of the a/c headache in the engine compartment (crowded) and the heater core nonsense. I obtained a non-a/c heater box, plenum and controls and welded up the firewall holes for the a/c lines and had to cut new holes for the newly relocated heater core hoses.
Why, oh why did Ford design the heater box the way they did on our Cats? Figure on at least a day, and a lot of patience to remove your heater box. Remember that some of the nuts, bolts and screws holding it are accessed from the engine compartment side of the firewall down low. A real joy to get at! Remember that your old heater box will be very brittle with age and will crack and break at the least ammount of forcing!
Good luck!
P.S. My old full-size Ford was so easy to swap out heater cores -- four screws holding an access panel in the engine compartment right below the wiper arms. Remove core, replace. Boom - Done! Five minutes, tops!