Indeed, aux fuse block would be ideal, and usually those are set up for both constant power and ignition power taps, but....
Have you found the blank ignition-controlled power on the bank next to the steering column?
I just used a male spade connector into the fuse slot. Works good if you want the CB to turn off when the ignition is off. For the constant power, I'd run straight to the battery (through the drivers side grommet by the e-brake cable), which would be the cleanest source for your CB. Keep in mind you don't need that if you connect the ignition power - I think it just allows you to program stations and have memory when you stop/start the truck. The CB will run with just red/black hooked up, but every time you stop/start the truck, you'll have to change your channel back to where you were.
Another place to get accessory power is the cig lighter, but I think the existing blank in the fuse box is easier.
1976 FJ40 - Green
1996 FZJ80 - Two-toned goodness
2005 KTM 450 EXC - Orange, of course