I'm not such a big fan of IRS from a performance point of veiw. particularly the Datsun semi trailing IRS setup like in the above pics:
1) its heavy
2) poor geometry control - ie toe, camber, and roll centre though suspension travel all changes alot, which means all sorts of nasty things happening during cornering - bump steer, weight jacking etc.
3) excessive squat geometry too

the 'fix' adopted by most circuit cars is 1200lb springs to eliminate as much movement as possible. I think the wheel rate of that is a smidge over 300lbs

much easier and better to simply set up a good leaf spring live axle under the back of the 1200. It will work beter and be lighter.

oh, and all those extra unis soak up some power too.

but if you are after comfort or the camber look (which is not actually what you want for the rear for good performance/handling)