You do not need to remove the pulley on the ITW or Thompson pumps it you're changing the reservior seal or rear output seal where the high pressure hose connects.There are three places for a Thompson pump ('67 to ?) to leak. The front seal, the canister or reservoir seal, and the rear/output gasket/seal. To do any work on theses, you will need to remove the pulley, before removing the pump from the mount. The reservoir and rear/outlet seals are easily replaced after popping off the canister/reservoir. Reassemble after cleaning and lightly lubing.
The front or shaft seal will require disassembly of the pump itself. Unless you're pretty confident of your abilities with pumps, I wouldn't tackle it. I'm a former AF Hydraulic tech, I didn't disassemble mine; because the instructions weren't complete enough. If it's leaking out the fron, get a reman pump.
These pumps aren't hard to work on, I redid mine with seals, although since it was only leaking and not noisy or otherwise bad I didn't disassemble the cam pack and replace the vanes. They are messy though but like was mentioned, if you're willing to do it then you can save a bit of money. I detailed and repainted my pump when it was apart, here's a link to my Photobucket account where you can see some of the pics Renee_237 - Photobucket - Video and Image Hosting look towards the bottom of the page.
Bleeding the pump is easy too. Once the pump is back in the car fill it with Type F ATF and start the motor, keep adding fluid until it's full again. Now turn the wheels from lock to lock several times and check the fluid again and add as needed. Go from lock to lock again and recheck it, you may have to add more Type F but the air should be gone now.
When you remove the lines from the pump have plastic sandwhich bags ready and rubber bands. Once the lines are off put the bags and bands on so the fluid doesn't leak out everywhere. Keep the lines elevated too, not pointed at the floor. Some fluid will leak from the pump itself too.