I searched for awhile to try to find instructions on how to do this and the manual I have doesn't specify exactly how to.
How to remove the rear bumper on a 1974 Porsche 911:
1) Unplug license plate lights so wires are loose... this takes about 2mins with a flathead screw driver.
2) 2x8MM bolts on the accordion bumper trims on each side are accessible from underneath/behind. Take the two on each side that are furthest to the back so the accordions stay on the car and not on the bumper. After you get the nuts off the back you must push the accordion away from the bumper so the studs release.
3) Remove small caps on the thin rubber trim on the back bumper... one on each side. Don't take the big one off... leave it alone. They will be about 6MM wide and blend in... there are phillips head screws underneath that have to be removed as well. Once the screws are backed out, peel the rubber strips carefully away from the bumper only until you see two nuts on each side... these do not come off entirely in this step. Clean up before reinstalling so you don't have damage to the paint from sand etc.
4) 2x13MM bolts on each side are the last step for the bumper to come free. Be careful to support the weight of the bumper as you get to the last so you don't drop it and wreck the paint.
Installation is the reverse of removal.
I haven't made as much progress as I would like to this point but I think it's more important to get everything clean and to be thorough in documenting as you break it all down. I've been through a roll of blue shop towels, 50 nitrile gloves, and 3 cans of cleaner to get to the point you see below. The amount of filth that has come off the engine thus far is incredible and there is a lot more to go. There was grime packed in everywhere...
My parts order from Pelican is in and the new headstuds are set to arrive on 10/3/2012. I removed the lower valve cover on the left bank and checked all the studs. Surprisingly there are no broken studs on the left side of the engine at all. I have more cleaning to do tomorrow before I may safely open the lower cover on the right bank to get the final count. I'm keeping my fingers crossed...