77, the bit you need is a "security" torx bit (or something like that) it is basically a normal torx bit with a small indentation in the centre to fit over the nipple in the centre of the screw.
I don't see how you could change the regulator without removingthe plenum from memory (been a while since I changed a regulator).
I have usually replaced regulators with the rail off the car but from memory it should be possible if you loosen the runners at their base where they connect to the manifold so they have some play in them and then remove the plenum itself.
At any rate, once you get that far it isn't that big a deal to remove the rest of the stuff if you need to...
Lastly, depending on the adjustable regulator you have you may need to clearance the underside of the plenum to allow it to fit.
