If the box in your car looks like mine you can get at the insert from the engine side. Even if you unbolt the box from the inside I don't think you will be able to flip it around because of all the wires hooked to it. It's also possible the threads may be stripped out of the insert.
The fuseblock should move enough for access to the disconnect.
I have removed a few of the disconnects from junkyards for different projects.
The disconnect is the same from 68/9 to the mid to late 70s
the fusebox changed in that time from the glass fuses to the ATO.
I just put one of the early 80s GM disconnects in my Falcon.
Easier to wire a car without having 1000 feet of wire to deal with.
Ford harnesses are way too pricey and none of the aftermarket
provide a disconnect at the firewall for a FOMOCO car.
Also---if the firewall side of the disconnect proves to be damaged beyond repair, it is not a big deal to to remove the terminals and replace it with a good one.
Some good lighting---being limber---and a lot of patience will be required tho.
This same block that you showed can be found up to at least 75 in the Chevelles and probably most other GM cars----pickups are different.