If you switch from auto to 4 speed you need to find manual trans pedals, z-bar, downrod, frame side z-bar bracket, bellhousing side z-bar bracket, bellhousing, transmission, shifter rods-levers-mechanism, shifter handle, and a 4 speed floor hump. Cool thing is Brewers performance has all of these parts in stock. Check out their website at
http://www.brewersperformance.com/ If you decide to leave the car pushbutton automatic you can either find a big block pushbutton transmission or use a later trans with the Imperial services conversion cables
http://www.imperialservices.net/ to operate the transmission or block off the pushbutton on the dash and go to an aftermarket automatic trans shifter on the floor, from B&M
www.bmracing.com or Hurst.
www.hurst-shifters.com/ Definately upgrade the fronts to disc brakes if you decide to do the rears after you swap out the tapered axles out of your car for some later style bolt-in axles you can either go with the original 10" drums or upgrade to 11" drums or convert to disc brakes from a bunch of different companies check out dr.diffs website at
http://www.doctordiff.com/ or scarebird classic brakes makes disc brake conversion kits for a bunch of different cars including older mopars
http://www.scarebird.com/