2012 mk4 rear suspension options

[Homebrew]

I currently have a 2012 MK4 with 4 link, donor parts. I don't see ffr offering any IRS other than the new 2015 option. I don't believe this option will work on my frame. Do I have an option to retrofit an IRS or should I just go with the retrofit of 3 link option? Third choice is simply upgrading all the shocks to adustable coilovers?
Most of my driving involves spirited motoring in the mountains of WV, dodging dear. In addition, a couple autocross events per year.

[edwardb]

The frame you have is either solid axle or IRS. Whether the previous IRS version which was selling in the 2012 timeframe, or the newer 2015+ Mustang option sold now. If you can fabricate and weld, it's possible to make the swap. It's been done. But there's nothing from Factory Five that directly supports it. Your donor 4-link would be way better with aftermarket parts including coilovers. Switching to 3-link would be better yet.

[Gordon Levy]

My 5 link is the best option if you plan on keeping the live axle.

[edwardb]

X2 on Gordon's 5-link. It's a great choice as well. Had it on my #7750 build and was very happy with it.

[Homebrew]

Right now I'm leaning towards the 3 link because of the Challenge series history of durability. In addition, I wouldn't trust my welding on the rear end of a fast moving car. On one of my slow moving tractors , sure.
Honestly, I forgot about the 5 link option.
Will I be able to install the 5 link with the body still on the car? Any welding or fabrication needed for the 5 link?
Thanks for all the input. You guys have me thinking.

[Gordon Levy]

The reason we designed the 5 link is because we broke the 3 link several times. On the MarkIV it need to be welded in and can be done with the body on but it takes a bit of work.