I measured 3 different Jeeps and got 3 different measurements. I measured just below the head...straight back to the firewall. 1", 1-1/4", and 1-3/8".
My next measurement is not scientific. I reached down with my finger and estimated the distance between the linkage and the firewall...just before the firewall ""opens up" to allow for more bell housing clearance.
CJ3A--My 1" reading from above is matched with 1/8" clearance between the linkage and the firewall.
CJ2A--My 1-1/4" reading from above is matched with 3/8" clearance between the linkage and the firewall.
CJ3A--My 1-3/8" reading from above is matched with 1/2" clearance between the linkage and the firewall.
The only Jeep where the tub has never been removed is the CJ2A.
What all this means, I can't really say. I can suggest that the CJ2A measurement is more likely to be the original target, but you can still make it work with just 1" between the engine and the firewall. If you currently have 3/4", I would try shifting the tub an additional 1/4" to the rear and see if you can get all the attaching points to work -vs- cutting the firewall. If the overhang at the rear bothers you, you could remove the bumper and add some spacers. I don't believe you'll have too much to worry about with other items that penetrate the tub ( gear shifters, brake/clutch pedals, etc. )