ill toss my two cents into the ring....
my elec gauge came with the obligatory 6' of capillary tubing (i think that is standard). i dont think i would have made it from the stock sender location to the pillar. but i mounted mine in the middle console, so i went through the firewall near the center of the car.....
i would find where you want to go through the firewall (hopefully near the brake M/C) and plumb a 6' piece of string through, to see if it will make it. that is easier than plumbing the gauge and line to find it does not make it.
you could try searching for post of pics from Mike (Mustang5L5). i would do it for ya, but i get killed searching pic threads with dial up. he has posted pics of his gauges. if you see that he has a temp gauge on his pillar, it must make it (im pretty sure he runs mechanical gauges).
sorry im not much help. good luck.