I use to work for bmw a few years back and the e46 rear subframe mounts are notoriously weak and if you're going to run that sort of power you must reinforce them. Otherwise you'll crack the mounting points and that's when things get messy. Basically the n/s rear mounting point cracks first and when that fails the o/s front mounting point cracks. A lot of people refer to this as the boot floor but technically its a large structural part of the body work that sits underneath and in front of the boot floor. I'd say the majority of e46 M3s out there have had this part of the body replaced under warranty.

You'll need to reinforce the two rear mounting points by using bmw's repair method which involves injecting that area of the body with their expoxy foam which sets like concrete and prevents the mountings from flexing.

Or cut away the upper part of the boot floor to reveal the sub frame mounting points and have a custom cage tied into the points and then weld up the upper boot floor. It's a bit difficult to describe but there's loads of info out there.

Anyway, sounds like an awesome project. Just thought I'd give you the heads up on the rear floor issue.