As SSedan said. You don't have to cut any wires. There is already a handy source of fused power going into the brake switch -- the little plunger under the dash that the brake pedal arm presses against when the pedal is pushed.
This switch has a hot feed going in and two male tabs for power out - one tab is normally open (doesn't get power until the brake pedal is applied) and sends power to the brake lights. The other tab is normally closed (supplies power when the brake pedal is not pushed down) and it used to power cruise control or power to the TCC lockup.
Start with a wire with a female connector crimped on and run it from the normally closed male connection to one side of a toggle switch. Connect another wire to the other connection on the toggle switch and run it out through the firewall to the TCC connection on the side of the trans. That will provide power to the TCC if the brake pedal is not depressed and the toggle switch is in the On position.
Like I said earlier, you can also run the wire from the toggle switch thru a vacuum switch before going to the trans if you want to have the feature that disconnects the TCC lockup under high throttle (low vacuum condition).
You get the vacuum signal from one of the nipples on the carburetor -- some like ported vacuum which disconnects TCC if you take your foot off the throttle, others like manifold vacuum which remains high when you let off the throttle and keeps the TCC locked up as long as you don't brake.