You will have to update your front suspension componets as well. Ball Joints, springs, spring saddles, etc.. Those original parts are not made for that much weight, if you look at the part numbers for the L6 front end parts versus the V8 parts you will see the difference. I was going to install a "stroker" my old coup but the front end upgrades were too much for my budget.
I have spoken with one of the original engineers on the early Mustang and they did try putting big blocks engines in the orignal design but with the added torque it twisted the front of the car so bad it would crack or pop out the windshields of the car. So with in mind you would also have to upgrade the sub frame to help with the "Torque Twist" of the front end.
The differential, rear springs will have to revamped as well considering the more muscle being put to the rear tires.
Don't get me wrong it can be done but you just can't drop it in and expect to have no problems.