If the t bird fan will fit since it is bigger, I'd run it. The t bird fan pulls more air by quite a bit. The blade on the t bird fan is 18 whereas the Taurus fan is 16" iirc.
for all you guys with 4.0 yj's and possibly tj's, i found a way to gain an extra inch of room between your water pump and fan set up. while installing the taurus fan, simply change your water pump froma yj pump to a xj pump(regular cherokee) and pulley set up. the yj water pumps are 3.5inches from mount surface to the tip ofthe fan nipple. the xj fans do not have the nipple and have a shorter shaft. when i get back from va i am going to try and mount my tbird fan with that extra inch of room
I found a Lincoln fan (3.8L Towncar) that was the same shroud dimension as the Taurus fans, but was 17" dia. From my testing (that fan & 2 Taurus fans) there was very little difference in RPMs between them, so put the larger fan in the Mustang & kept the other 2 for mine & Taters Jeeps... both sporting flexfans & no shrouds, so figure they should do fine.
From what I've read on the ford truck and bronco websites there is really no difference between the mk8 fans and the t bird fans. A taurus fan is supposed to pull about 3500-3800 cfm on high and a t bird fan should pull 4200-4400 cfm on high.