Honestly, If you want to make a lot of power and not worry about your rear end I would go with a 9" ford rear and a truck arm. The 9" will let you go from mild to wild and you won't have to change out the rear. Talk to Steve at
www.hotrodstohell.net. He has a vast knowledge base of truck arm suspensions. His friend Gchandler has a rippin' chevelle that can smoke most anything on the road and he has a truck arm setup. I think IRS is better suited if you are going to drive mostly street and not put more than 700 RWHP on the car. This is the case with me but I am planning on doing a full frame to go along with it so I won't have any packaging issues.
If I'm not mistaken, Watts device keeps your rear end from moving side to side.
As far as Heidts, they seem more appropriate to pro/street and drag cars. You know, straight line performance. And they're pretty too! I want to build a high performance sports car with a 2nd gen body('66 Chevy 2) so I am going to go with a front end with corvette components. Again, Wayne Due(
www.waynedue.com), Art Morrison(
www.artmorrison.com), or Jeff Schwartz(
www.schwartzperformance.com) can do this for you if this is what you want your car to do.
Try and buy a book called "Chassis Engineering" by Herb Adams. It will give you real good perspective as to how and why things do the things they do and what you want to look for in a suspension and chassis. Good read although I had to skip some of the formulas.