Mike, the so called rear firewall is mostly flat but it's not vertical. Instead it slants backward at (I'm guessing) maybe 15 degrees between the floor and the back window. So clearance at the floor is quite different than clearance up at the bottom frame of the back window. Clearance would likely be measured from the engine front, which itself is extremely irregular.
Interferences are from the lower rear window framing to the distributor cap up top, the protruding water pump shaft & bearing support and especially the alternator & its pulley in middle. All of these are addressable individually. Ford Performance used to sell a 'Street Rod" water pump for the 351-W that is 2-1/4" shorter than stock pumps, and can be made to bolt on a Cleveland. It was once sold with its own short pulley but that was discontinued decades ago, so such short reach pumps will need custom pulleys and adjustment of the alt & AC belts..There are racing water pumps (Davies-Craig) that could also be adapted.
The alternator can be mounted on a stock wide-body alt/AC bracket in the area where '85-'87 Panteras had a smog pump. This rig uses a belt up from the inner pulley sheave on a stock A/C pulley, which itself is driven by the stock pulley sheave from the crank. And one vendor (D Quella) builds a bracket to mount the alt offset far to the front-right-outside, behind the non-removable part of the rear firewall. A bit complicated to change vee belts on this rig but do-able.
My choice 20 years ago was to swap the engine's lower motor mounts (they were drilled asymetrically due to the block's cylinder offset). Doing this shifts the whole powertrain backwards about 7/8". The shifter rod need only be readjusted and a 10-qt pan still clears the mid-mount crossmember under the engine. All the cooling lines & wiring still fit as stock. The only adaption is for the rear ZF mounts, and that can be done by bending the rearmost mount tabs into a shallow 'S' and making 2 new spacers. Combine some of the other li'l mods and you have a 'flat firewall'. Plus, the engine can now use a more modern big distributor cap,.
Some of these mods almost or fully remove the need for a bulge in the rear firewall cover, hence the term "almost-flat-firewall". Once the alt is shifted, the need for a bulge mostly disappears, and the right seat adjusts back so far, even tall passengers' feet can't touch the front toe board! I don't understand why ALL taller British, Australian and Japanese Pantera owners (with right hand steering) don't use some of these mods more often just to be more comfortable driving.