Im not sure on the cut off date,but i think the 1" rotors were used on some apps 80 & up. Most of the older stuff that i have been into has had the heavy rotors. i have seen that job done several ways,but the easiest....)if you dont have spindles already),may be to pick up a set to bolt in for your app. I converted my 69 several yrs ago,when there werent many parts available. You will find that the lower a arms have a different steering stop,& will shorten the steerin radius(i used 71-72 lower arms),& have 75 spindles(cut coil drop). I set it up to use a bell Tech spindle(73 up was available).......talking with bell tech,they said use the ball joints,& outer tie rod for the 73 -79 spindle. There have also been some differences noted in the center link,so if you go for 71-79, you should be ok there. lenghts are also different on tie rods & sleeves.....so i can see why some guys are swappin a complete crossmember(all the parts are off 1 truck). Here is a link to spindles for your app.....
http://www.classicperform.com/discdrop.htm