Perhaps I misread, but I thought you said "As far as I know, nobody has yet to actually install the Taurus rear calipers on the Mustang yet with any setup, let alone with the ATS fronts". Based upon the pic it appears you have. I assumed the 43mm was the same as the 44mm, the latter which did not fit either the 38mm caliper cage or the axle flange.Not sure if you misread my comment, or was just adding additional info.
I was referring to the 93-98 Ford Taurus 43mm caliper. It's interchangeable with the 38mm unit. Here it is just test fit on my car with the red 38mm unit hanging in the background.
The only reason I was interested in the 44mm version was for the bigger pad (D347 profile), not the larger piston per se. What profile is the 43mm? Same as the 38mm?