Questions are good you learn stuff!
Unless your restoring your Xp dont waste your time using Xp parts as you will get better parts for less money using later model components such as Xc stubs, calipers, disks and a Vacume booster and Dual circuit master cylinder and 14" rims.These are easy to source and cheaper to service You will have to sort out the stud pattern on the diff Which will only take more power for so long.
I too was concerned with bump steer using Xc parts but it was a not really a issue with the shelby modification to the top suspension arm and a non spec wheel alignment .I reckon the stopping power and inproved cornering outways the only negative ive noticed which is a lightness in the steering at high speed. But this could come down to the Xc power steering thats also been fitted.
RRS which Raptor is talking about if You have the money also sell a bump steer correction kit which moves your inner tie rods into the the correct position to the outer tierod point on the stub axel.
Cant help with the 2v head swap though but if your planning on that using a 2V head the 4.1 engine its attached to Cause you cant beat cubic inches.I doubt any one make an off the shelf headers any more .When i bought my first Xm i threw an alloy head crossflow and C4 into it,cost me $200 dollars to get the drive shaft sorted and T bar shifter and Xc throttle pedal and i was rocketing down the road with really under powered drum brakes. Really and I mean really sort out the brakes before messing around with more power with these early falcons!!
Have you checked out www.xpfalcon.kicks-ass.net
as it is an awsome site and has more infomation you could imagine