I did similar to Dremu, measure the opening in the center, and two arbitrary points on either side of center. I used a sharpie to mark the points where I measured.
It's not the 'A' pillar itself that sags, it's the cowl. I would brace from the lower area of the door opening at the striker end, to high on the door opening at the hinge side...then begin the cutting. I would add a cross brace from 'B' pillar up high to other 'B' pillar on the other side. I had issues with spreading that I didn't find until after my cage was in. Took some focking around to get most of it, but due to the cage I couldn't get the last 1/2".
I would not tie the cage to the 'A' pillars...as I said it's not the 'A'pillars that sag but they do move around a little. I would consider adding a tie in point from your front cage bars to the cowl area (under dash about knee height)
It's not the 'A' pillar itself that sags, it's the cowl. I would brace from the lower area of the door opening at the striker end, to high on the door opening at the hinge side...then begin the cutting. I would add a cross brace from 'B' pillar up high to other 'B' pillar on the other side. I had issues with spreading that I didn't find until after my cage was in. Took some focking around to get most of it, but due to the cage I couldn't get the last 1/2".
I would not tie the cage to the 'A' pillars...as I said it's not the 'A'pillars that sag but they do move around a little. I would consider adding a tie in point from your front cage bars to the cowl area (under dash about knee height)
With my cage already in, all I could do was suck in the box sides in as tight as possible and weld in some bracing to hold it there. I'm sure your truck is more solid than mine was, but at least throw a tape measure on there and check it...