[ffindling]

There are a lot of IRS options you can go
with. It basically boils down to what you
want and how much you are willing to spend for it. Jag IRS assemblies are popular and if they are properly built, 500 hp is not a problem assuming you are not
going to drag race the car with sticky slicks. Jag rears with the exception of E-type rears have to be narrowed for most Cobra applications. It's not a job
for novices, but there are shops who can
do it for you. Some Cobra manufacturers
will narrow customer supplied Jag rears
for their customers. So if you buy a package from them, they'll narrow/overhaul a Jag rear you send them as part of the deal.
Vette rears have been used in Cobra
builds, but like Jag rears they usually have
to be narrowed as well. Hunter sells Cobra kits based on C4 Vette suspension. Corvette suspensions have the advantage of being relatively
cheap and plentiful.
8.8" T-Bird IRS is another option, like
Vette stuff, cheap and plentiful and a
much more modern design to boot. It
has to be narrowed as well, but there
are shops who can do this.
Then there are custom shops who build complete IRS assemblies most of
which follow the Jag model. Not cheap
by any means, but they are for the most part real works of art. They can
supply them with either inboard or outboard brakes, pin drive hubs, leading or trailing links or watts linkages for extra bracing of the uprights. They can build them in just about any width you want. A lot of these custom setups are based on 9"
Ford centers. Nice stuff, you just have to be prepared to fork over for it.
You could also create your very own
custom IRS system if you wanted. Lay
down a design using parts of your own
choosing or design. There are a number
of suspension gurus here who would be only too glad to help.

...Fred