Don't cut into the loom.
If your 70 is like mine, under the bonnet, in the front right corner by the air cleaner, is the fuse box you are looking for. Mr T, in his generosity, put it there just for you to use for whatever you want. Under the lid you will find an earth (E-) terminal and three positive terminals, each with its own fuse; one on the right as you stand at the front of the car (B+) is permanently live; one in the middle (IGN) is live when the ignition switch is on; one on the left (ACC) is live when the ignition switch is in the ACC position. Run a wire from whichever one suits your purpose back through the bulkhead and up under the dash to wherever you need it. Put in a junction box under the dash if you have lots of things you want to run off it, but fuse each accessory individually.
My car is 1994 so this fuse box is situated conveniently for extra lights, it dates from the days before everybody wanted navigation systems, cameras, and all that screen-based stuff we take for granted these days. If your car is a lot younger maybe Mr T moved this fuse box to a more convenient place for all that gubbins, perhaps under the dash. Have you got a wiring diagram for yours?