Well I'm planning the same thing although I haven't started ripping things apart yet I am just getting my facts straight before I start
So one of the questions some people may have is WHY? and WHAT ABOUT A QUAIFE?
Well in short this route is cheaper for below reasons (asuming your getting a used M3 subframe)
If you install a Quaife you will have a great LSD but everything else is 335i. If you are running stock power then IMHO this works great
BUT most of us are trying to break the sound barrier RIGHT!
M3 complete subframe gives you
1, Amazing LSD
2, M3 bushing all over the rear
3, Bigger Stronger Drive Axles for the bigger power
4, Bigger Stronger Prop Shaft for the bigger power
5, Better sway bar or ARB as we call them over here
6, Rear breaks get upgraded by default if you keep them on
Add this up and for the same price or less the Quaife isn't such a great deal
So HOW do we do this and remove all the smoke from this subject.
Well if I could I would create a KIT and sell it to the 335i fraternity who want to mate a complete M3 Subframe transplant. HE WHO DARES WINS
What would the kit have (Oh! and pleae add to this idea)
Part 1, Prop conversion kit which contains the front prop shaft which mates to the M3 rear prop shaft and MISC parts etc..
Part 2, Optional Front BBK to compliment the M3 bigger rear brakes (the 3rd part is no longer required if you go with this option)
Part 3, Adjustable camber arm? so we can use our 335i hubs, disks and calipers
Some Facts that I have picked up so far (Thanks to Idnan, what a gentleman!)
M3 7DCT Diff FD 3.15
M3 6MT Diff FD 3.85
335i 6MT Diff FD 3.08
335i 6AT Diff FD 3.46
(FD) means Final Drive for those googlers
So when looking at this data, 335i guys with 6AT trans have there pick of both M3 diffs as they are 0.30 between the two ratio's. BUT IMHO 6MT guys should use the 7DCT M3 diff or else the final drive will be way to short and 1st gear will be unusable
I've opted for the 7DCT Diff as I feel with my power mods, 1st gear is too torqued and I end up starting with 2nd. With this in mind the drop from 3.46 to 3.15 will give 1st gear slightly longer gearing but not as long as the current 2nd gear. Once the 6AT is rolling it makes no odds as the Torque converter will sort things out.
Maybe it might improve MPG on highway cruises
So another important factor is the Prop Shaft modification which is playing on everyones mind.
Which M3 prop to use and how to measure and cut weild it
I have noted that the M3 has two sized props. I believe 7DCT diffs carry the longest prop so maybe for the sake of a cm here and there get the longer one
How to measue it? Well I'm no expert but my theory is to offer it up to the car decide which end to cut (I'm siding with the Trans end) and cut as close to the end as possible. You will then need to use your 335 prop as a donnor to mate this to the 335 tranny
I would measure with some form of solid wire which has zero stretch and clamp it to the diff and then pull it over to the tranny. I would also cut less which may mean more cutting but also means you have options if you cock it up.
Weilding it? well obviously get a professional prop shaft company to weild and balance the prop but until meanwhile I would tack the cuts together like a prototype so you can install it to the car and turn the wheels and see if your prop is fitting in all the right places.
Well good luck to all attempting this and once my project is underway I will detail my steps and offer up real measurements for those of you interested with pictures and all