mate here is a link you might be interested in Shane's EL Futura
this guy has put in IRS from an AUII VCT XR6 as well as front suspension and VCT motor. This would also be possible on an EA-ED as the AU floor pan is the based on the same platfrom EA26.
I wouldnt even bother considering the BA as the suspension towers are different and there are major chassis changes in the rear. Plus the AU IRS is pretty good. Better than anything holden have ever put out there. No crappy squat as you get from the inferior semi trailing arm setup as on the VN-VZs.
I see that you want to attempt IRS on a wagon. Well thats one big job considering there is no falcon wagon with a coil sprung axle let alone IRS. You will need to do some serious re-engineering and I mean, more than your EL is worth unless you are an engineer yourself and write up your own compliance for the modifications. However the wagon is based on the LWB (fairlane/LTD) platform so there might be a way of adapting coil sprung rear from AU fairlane if they got IRS treatment as I only know of XR8, XR6 VCT and fairmont ghia.
anyways, hope this gives you an idea of what your in for.