Ok, take this for what you will but I've been researching suspension parts for mustangs since about 97.
The watts link is a "superior" method of centering the real axle when compared to the pan hard bar, as the watts will keep the axle centered and the BHP moves the axle from side to side with suspension travel. But the effect is minimal and actually counteracts the effect of engine torque on the rearend (drive shaft torques the rearend right and the PHB torques it left (or visa versa, can't remeber which). So all in all, I wouldn't pay for the watts link over the PHB (simply beef up the stock PHB setup). I would simply put my money elsewhere.
That said, the lakewood unit looks of the proper design. You want the pivot point attached to that chassis, as it is, and the axle brackets are probably sufficient. If it's rigid enough is another question...
Dan
P.S. You'll have to reroute the exhaust most likely as well, a pretty big downside.