When trying to decide on solid vs. independent rear suspension it comes down to to basic factors; cost versus performance, with ride a major factor in performance.
Solid axles are definately less expensive and more durable. (That doesn't include the U-joints.) If you'll be drag racing only and driving on roads that are smooth as a pool table, go with the solid axle, providing you don't care about the ride and handling of a Cobra. But be aware that you've got to use relatively stiff springs to restrict wheel travel. With a short driveshaft, there will be a lot of driveline angle change with wheel travel, thus whipping of the driveshaft and U-joint wear.
On the other hand, an IRS is far superior to a bouncing axle in terms of ride and handling. And there are plently of them available such as the T-Bird/Cougar/LSC Lincoln setup with disc brakes attached. You can go with a much lower rate spring, which would give your car a smoother ride. Then there's the original Jag setup to consider, too. I have a friend who installed a Vette IRS in his Everett-Morrison, but I would stay away from that one.
For additional information, go to ERA's web site:
http://www.erareplicas.com/427/427sc.htm#backfromfsusp . It will provide line drawings and information about why they went independent as did Superformance, Kirkham and others. As for why other manufacturers went the solid-axle route, it was to use the Mustang as a donor car, cost and/or ease of construction.
I hope that helps. By the way, I just pulled a complete Jag setup out of an '88 XJ6. Anyone want one?
