That is the beauty of using the quadrant.   No matter where the pedal is located, the cable still goes horizontal from the top of the quadrant. 
But, just so you have a standard to go by, place the clutch pedal in static, up position and measure from there using a straightedge.

As a parallel idea...Does the cable clutch give more room for power brakes?  I am considering PB and wonder if the scope of my thinking needs to expand.

here is what I am considering doing to locate he hole. clutch pedal all the way up, a long 1/16" drill bit on a right angle drill, lay the drill bit in the quadrant and drill the hole in the fire wall. I realize that drill bit might be to long but it can be cut down to a length that could work, or at the very least mark the fire wall location. just an idea I am considering.
 

lowercasesteve wrote:

As a parallel idea...Does the cable clutch give more room for power brakes?  I am considering PB and wonder if the scope of my thinking needs to expand.

The MustangSteve clutch cable system does allow for the larger Fox body booster really helping in the power booster department.  With that said, once my new shop is built, this spring I'm continuing work on developing a MustangSteve hydraulic clutch system.  In conjunction with that will be a whole new power brake system platform. 

trying to put this on my 64 falcon, here are the issues i am having with the billet brace. the pinch weld is not deep enough to drill holes thru to mount the billet brace. the pinch weld is not flat, the drop is over 1/2" in the length of where the billet brace would mount.
 here is what I am considering as a fix. mount a plate on the fire wall, to distribute the weight of the action of the clutch, so the fire wall won't crack. mount the billet brace directly to the fire wall horizontally, after drilling new holes. since the billet piece is 4" long, is 1/8" steel plate  say 4" larger than all the measurements, both height and width sufficient

the lip does not come put far enough to even drill the bolt holes for the vertical mounting of the billet brace. add to that, the distance  of where the 1/2" is located is not even close to the pinch weld. falcons are just a bit different in the cowl areas as to they are in the mustangs.
 

Typically a spacer is required on Falcons to lower the billet brace where the hole will line ip with the quadrant.  Some customers have welded extra metal to the pinch weld. 

Bottom line:  make whatever you use fit solid against the firewall where it cannot move.  Clutch actuation forces are quite large, so make it strong. 
My 66 has a length of angle iron with a large washer welded to it, bolted to the pinch weld.  This was my prototype design before the aluminum adjustable block was designed.

not much in favor of welding on a painted car. I have been working on getting the fly wheel, clutch and bell housing in. just don't feel like installing the trans tonight. still considering how I want to approach the fire wall and bracket. since I don't watch any professional sports it will give me something to do tomorrow.