The '87 rear will also have coils springs in the back..not sure on the Merc. Your Mustang has leaf springs. So, you'll have to deal with that.
As for the T-5, find out the number on the data plate, then compare it to this chart:
http://www.geocities.com/MotorCity/D...66/t5data.html
You'll learn how strong it is from that chart.
As far as the rest of the stuff..good luck and let us know how it goes. We'll help where we can. And again, with your current motor/trans info I requested, I can lay out the T-5 swap into the 6 easily for you.
Al
------------------