Similar here, cut away the original tower but I made some vertical reinforcements, not sure if they are really necessary but the full weight is now at the top. Also I used the original top hole.
No need to build your own coil-overs, I used a pair intended for a Golf from a Jamex coil-over kit that the seller only used the fronts from. There is plenty on offer, new/good used/cheap/expensive. These Jamexes have the right hardness/harsness for me, adjustable in rebound but the soft setting damps the springs properly for the 2 years I have them, later I can adjust them for wear.
'08 GT 3.2, Sud '82 QO, '76 Ti, '75 L, Montecarlo, 308.