Here's my second crack at this today. I've thought about this and it depends on what your objectives are in the end. I'm going to just pitch this out and we'll see how it goes. I've seen most modified 122's over the years and things like removing the floor and starting over have been done. Grafting the top half of the car onto another car has been done. I think it comes down to build philosophy.
So you have a HP goal and want IRS. Great. I've just said it isn't easy because you have to change everything to meet that goal. Most wouldn't think of that as easy.
I think it breaks down into two groups that finish. We won't talk about all those projects that end up for sale or just abandoned.
1. Hot rod builds. With a little cutting and welding and a sledge hammer, anything will fit into anything. It may not be good, but it'll be done. And often it is good. Nothing wrong with this approach (not going to discuss high dollar builds...wrong forum).
2. Design change builds. Cars are designed around many constraints. These nuts want to change the constraints. It's not going to be good enough to be good enough. They want things optimized. They want all the grip they can get and turn that little car into something it was never meant to be for no good reason. These builds/builders spend a lot of time with a calculator relative to time with a welder. Maybe they want to fit the car into a specific series or have rules/regulations to target. Who knows.
So by example, my IRS install seen through these two lenses.
Hot rod: The idea of an IRS is great. It must be better because it's independent. Travel to the JY to find that no sane car maker in the past 30 years has built an IRS in a car with a rear track of only 52"...but didn't Jaguars have about that with the E-Type...and wasn't it in a self-contained cradle with inboard brakes and integral coil overs and all that stuff?
BAM - You bet it was.
Clean it up and weld her in and we're rolling in style. All kidding aside, it's not a bad idea. And it's been done, by a guy with a lot of skill and I like that build.
Design: It would be nice to get away from the solid rear axle to enhance handling and optimize the rear of the car (hopefully to match the front). So we get the tape measure and head to the JY as well, but pass by the Jaguar, and head for something a little newer...probably multi link and modern. It should at least have CV joints. I ended up with this mess.
It's out of a Toyota Supra (MK3) - you get the Toyota differential, good geometry, and even some passive steer...but that subframe is rather specific to the Supra and won't fit in the 122.
So I built this:
Shortened it up 6"...then revised the entire rear half of the car to handle the load paths that the rear end creates.
Revised frame, cross members etc to handle the loads. Look at that snubber on the original frame...it's there for a reason. You have to work through all of those details.
Every detail that I can think of including reinforcing the front cross member attachment points inside and out, up and down to take the beating I'll give it when done.
I've hired a race engineer to work through the design and help me optimize the suspension. We've moved things around and this is version 3 of the design. So when you say that your parts all come from the same donor car...that's all well and good (mine come from two completely different cars) but it's all in the details. What I'm saying is there are a million details. Little stupid things that will trip you up and big things that will sink builds.
So to my way of thinking, putting an IRS in a car never designed for an IRS is not simple. It could be, but I don't want it to be too simple. Heck, putting 250 hp to the ground in a 122 means your doors are not always going to work right - I only had 135 WHP and managed to give the body a new set at a track day.