The stock isolators are going to be shot from age so new isolators is a good idea. Some ppl don't run them though. You are spinning because you have 3.73 gears and stock tires/suspension. Lower control arms out back, lower the car, good set of springs/shocks/struts, CC plates, subframes and a good set of wheels and tires and you should be in business.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.