On this page of my build thread is a picture that references 7.5" from the upper hole ctr to the large fender hole center, make sure to cut well below the lower hinge hole because there is a cage that holds that nut plate to the a pillar.. my replacement probably spans page 34-40 but i tried to document it as much as i could. Oh, i may not have posted it but i also found another pic showing that if you run a straight edge down the outside of the A pillar the bottom of it should be in about 3/4" In other words looking from the front.
http://67-72chevytrucks.com/vboard/s...595019&page=37
My advice is to brace the cab with cross bracing and then support the cab floor at the supports to keep it from moving, then peal away the outer rocker, then if you look at the a-pillar you will notice it molds inward to create a relief where the rocker was attached, then decide where your going to section the lower A pillar, preferably in that relief area, once its cut out you will need to fit the new A-pillar, then transfer and drill the support holes to it. Before welding it in if you have to replace the outer floor supports or inner rocker you now have a set reference point by having the A-pillar cut and fitted.
Here's another thread i saved for reference:
http://67-72chevytrucks.com/vboard/s...d.php?t=607099