I would remove driveshaft and headers (exhaust manifolds), install engine crane over engine to take weight off and allow motor and transmission mount replacement. Replace transmission mount.
Replace motor mounts thusly: bolt new mounts to motor loosely. Leave loose enough they have extra slop allowing mount to move freely side to side. Lower crane (engine) until the mounts are close to the same height as the frame brackets. Use taper through back of mount into one side frame bracket. This is generally easy to do since the engine is not attached at the other side. As you withdraw taper, insert bolt from front side of mount and leave in place with no nut securing it (just passing it through locks mount to frame bracket). Go to other side. Use crane and muscle power to move the mount into position over frame bracket. Again use taper from back side of mount thru frame bracket. As you withdraw taper insert bolt from front side as before. Once all bolts are in place, tighten the engine to motor mount bolts first. This will draw up the mounts into position. Put nuts on lower bolts (motor mount to frame bracket) and tighten. Replace manifolds (headers) and driveshaft.
Obtain motor mounts from quality source. These are no fun to replace and so you want them to last. Try Lakewood Muscle Motor mounts or buy OEM Ford parts. See John's Mustang for pictures of the ones you need.
http://johnsmustang.com/ClassicMusta...s/Motor_Mount/
http://johnsmustang.com/Parts/details-4097.html