Since you're not building a purpose built race car, then I would seriously consider checking the stock rubber bushings on all corners and if they look good , just leave it alone. Reason: These Mustangs were built with a certain amount of flex-compliance designed into the rear fourlink having ride comfort being the goal. When you step up the HP then you need to consider changes to the
suspension. Personally I would then consider that the torque boxes could be effected by the added power transfer. I welded my seams and let that rest as I have a Vert and it was off the chart to install plates etc.
Bushings:
The horse has been beaten beyond death concerning poly vs rubber bushings. It really comes down to what you intend to do with the car and how much you want to spend. Personally, on my car I have decided to stay all rubber as I am not carving corners and in the same don't want the risk of picking up squeaks and any more rattles from stiffening the car. Polys have to be lubed occasionally or you'll forsure squeak.
Another thing: Stiffening up the rear and doing nothing to the front will induce weight transfer issues that may or may not be felt but will be effected, as one end of the car will want to stay put while the other end will give. These cars were not of the best design from the onset, but they can be made to handle better but that is best achieved as a total package as opposed to piece meal. All this cost $$$