Randy, Congrats on knowing what to purchase way back in 69!
So having done this on a freshly painted SBC Camaro, it is not that difficult, just time consuming and you will be cussing!
I assume you do not have A/C, that is a WHOLE other procedure!
The core is in the heater box, which is inside the car. The bolts protrude into the engine bay and come out around the fan air box. It should be possible to remove the nuts (and core inlet gasket) from the engine bay side and get the heater box out, since the BBC heater hose inlets come straight out. If you have a console you will be fighting with getting the box out around the console. The console and heater control lines will hold you back.
If I were you, I would remove the heater lines 1st (removing the glove box, makes it easier & label them!), maybe the console too before attempting the heater box removal.
Then you should be able to get it out the heater box. The core is clipped in, remove the screws, then the core should come out. The core should be fixable (its just a little radiator). What I did was bought a repro core and had them replace the end cap and save the Harrison logo. I think they may be dated too.
Now is a good time to look at the seals in the heater box to see if they have deteriorated. Good Luck!
Removing the fan and fan airbox would require the hood and fender to come off