I would definately go ahead and make the swap to 4r70w. I did mine in March for around $1000-$1200, not bad at all considering that $600 went towards a Edge 3000 converter.
02' mustang V6 4r70w.....~$300 (found it at www.car-part.com)
Edge street 3000 stall converter...~$600
Misc stuff (oil, gaskets, tools, etc.)....~$200