Ultimately JJ, the combo of disk/disk and V8 in these cars aren't the
best with the 6 pot/V8 disk/drum setup being better, and this was
known back when these cars were new.

I've had 6 pot disk/drum XB/C/D's and 2 disk braked LTD's,  V8 XA
disk/drum, XC V8 disk/drum and the drum setup all had better pedal
feel and stopped better.

My brother has an immaculate ZH Fairlane with 4.9 and disk/disk, and
its exactly how I recalled my LTD's, wooden pedal requiring a fair
shove for heavy braking but fortunately as its a luxo car, it dictates
sedate driving and I drive it knowing full well its braking
capabilities. I wouldn't want to drive a 4wd braked hotted up XC V8
coupe with any vigor iethout long runoff areas :)

What makes them a marginal braking car nowadays is all the other cars
on the road have superior braking capability

Hi Jason, I'm coming into this discussion rather late, but from what I've
read thus far, you seem to be trying all angles.
As Atec said, look at adding a secondary vacuum tank, a'la XF and see how
that goes.
The V8, especially if she has a bit of a cam, will tend to lose a bit of
vacuum when you've hit the brakes quite hard
and used your supply up. The interior ventilation controls will also eat
vacuum if you have a leak in the system piepwork too.

Most Z / F series Fairlanes and LTDs I've been in have pulled up superbly,
and I find it odd that you're having so many troubles.
The FD / FE were markedly improved over the FC, when you consider the
upgrade with the XE spec calipers front and rear.
My old XE wagon, fully loaded with people. gear, and a 351 and assorted LTD
refinements would pull up very well, even with
the original 6 cyl spec drumbies on the back.
Mind you, it would only be good for a couple of decent stops!

Others have said that the brakes fitted to the big Fords were only marginal,
and I suppose this is true, but I tend to err on the 'better' side of
marginal, rather than the worse.
Perhaps someone may recall the spec's used on the Police chaser cars, as
they were improved from memory.
I think for the application you are seeking, it may be best to go to
something more after market, but even then, I would consider the slotted
rotors that others have suggested. I'd tend to steer cleer of the harder or
'sportier' grade pads, as I'd prefer to replace the pads rather than the
more expensive rotors.
That's just me though, as I'd get sick of changing wheel bearings and seals
each time.

As for factory improvements, try the following if you haven't already, (I
'spose you've probably heard of some of these anyway!)
1. Look at the integrity of the mater cylinder mounting at the firewall, and
also th condition of the pedal box. They DO flex as others have stated.
2. Double check to see that you DO have a V8 spec booster and master
cylinder. I 'm sure the master cylinder may be the same, but I think the
booster for a V8 is definately a larger diameter that the 6.
3. Upgrade to the XE spec calipers front and rear.
4. Use a good quality brake fluid, flush it out more regulalry if you intend
doing decent stops often. Brake fluid degrades with heat.
5. I'm unsure of this, but check also that you have the correct brake bias
spring fitted inside your master cylinder, there are different ones I
believe. I do know that the disc/disc spec is different to disc/drum, and
it's possible that you may have, or have been sold the incorrect one at some
stage.

See how you go mate, keep us posted!
Regards, Eddie.