for the front
suspension i would go with the V8 gt springs, they are rated at 480lbs/in os while they are stiffer than stock your ride wont be too stiff. remember that the six is much lighter than the V8 so you dont need as much spring rate. next while you have the
suspension apart do the shelby drop, lower the upper arm pivot point down an inch and back 1/8", to change the camber curve. dazecars.com has the template and drill bit you need to do the job. run a 1" sway bar on the front. i would also use adjustable strut rods and a roller bearing on the lower control car. also step up and install roller bearing spring perches, dazecars.com and opentracker racing products both have the needed parts to convert your perches, or you can buy the already done perches. i would also use a roller bearing to replace the stock idler arm bushing. again opertracker can be a help here.
as for the rear
suspension, since you have the six i dont recommend using a rear sway bar as that would cause over steer issues that can be severe. i would however recommend using V8 rear springs.
for shocks i would recommend using the kyb gas adjust shocks, remember to use a large, thick fender washer in the underside of the upper rear shock mounts to spread the load and prevent the shocks from punching through the sheetmetal. if you have the money though, run koni shocks as they are adjustable.
for brakes again since you have the six you dont need huge brakes, and 11" front disc brake will do nicely. scarebird, master power brakes, and others have a variety of kits you can use. my advice though would be to upgrade to V8 spindles as the six cylinder spindles were suspect to begin with. you can use any V8 drum brake spindle from 1965-1969 with no problems. if you happen across a pair of 1970 mustang spindles grab them as they have even larger spindles.
i would upgrade to an 8" rear end and 5 lug wheels.
as for wheels and tires, i recommend running a 15x6 or 15x7 wheels and 205/60-15 tires. with the six that is all you really need.
as for lowering the car, you can use a mid eye rear leaf spring, and take the front springs and cut the coils down until you get the ride height you are after. cut a 1/4 coil at a time.