I installed a 395 R18 5-speed in my S2 when I restored the car. It is a great improvement for
cruising, and the 4th - 5th gear spilt is quite close, making it great for overtaking. It has
a similar first gear, but a lower diff ratio, making it very quick off the line.  I'll run
through the major interfaces.

1 - bell housing to block - bolts straight onto the 1470/1565/1647 block. The input shaft 
for the transaxle fits straight into the bearing in the end of the crank. It also mates 
to the clutch drive disc. Too easy!

2 - clutch release lever - points rearwards instead of down. Removed the roll pins at
the throw-out bearing fork, reoriented, drilled new holes, reinsert pins - not too bad.
Note this lever is much wider than the original, and takes up valuable space for the 
gearshift tube running down the RHS of the engine. The 4-speed bellhousing and clutch
release lever do no fit the 5-speed, because the crown wheel in the 5-speed is much 
bigger. The crown wheel protrudes forward of the bellhousing-to-transaxle mounting face.

3 - throw-out bearing - use R18 

4 - R18 output shafts - too long. Need to remove the output shafts from your 336 
4-speed and fit those inside the crown wheel carrier - no problem. However, these 
shafts are further forward in the car than the original items . This may result 
in more negative camber in your rear wheels. I've got CV joints so I can't quantify 
the effect.

5 - R18 screw-in pieces that hold the diff bearing outer races. These have a 
different seal arrangement onto the output shafts. You need the screw-in pieces 
from your 4-speed box. Luckily they are the same thread, but the R16 doesn't have 
the outer peripheral O-ring like the R18. Use LOTS of sealant on the threads as 
you screws these in. The R18 has a bigger flange for this O-ring, so you may need 
to grind some away to get your roll pins into position.
 
6 - lower suspension link bracket. It will fit, but you have to grind away some 
additional ribs that grew out the side of the R18 box. Easy stuff!

7 - reverse switch - same place (RH side of trans), same switch

8 - rear cross member - I mimicked the cross member that the rear springs attach to. 
I made it bolt in, and it also helps to support the boot better. The outer ends of 
the cross member sit outboard of the chassis rails, so I had to chop away some 
bodywork, and make new body mounts. A bit fiddly but worth the effort. I made 
sure the rear transaxle mount had two bushes spaced well apart, to stop that dreaded 
S2 engine roll and twitchy rear end feeling I always seemed to have. Make sure the 
bushes aren't TOO far outboard, because you'll have no space left to run your exhaust.

9 - speedo drive - I bought an in-line reduction unit, and a few screw in pieces, which 
I then machined to fit into the R18 speedo drive. I went to great trouble to calculate 
the correct ratios to drive the speedo, and failed miserably! I suspect the 780 revs/mile
calibration of the speedo is not right. The speedo runs about 14% slow. However the tacho
confirms that I can cruise at 100km/hr at 3000rpm. This in-line reduction unit cost me 
close to $200 Aussie peso's, the whole transaxle was $100!

10 - gearshift - This was the biggest time consuming exercise. I made a new tube to run 
down the RH side (through the engine mount). In fact I made three before I got it right!
Eventually I ended up with a straight tube, and an engine mount with a much bigger hole 
in it. I also had to fabricate an ugly bracket to attach to the gear shift input shaft. 
This was all very time consuming.

11 - detent between reverse and 1st. Renault don't put reverse below 5th. It's off to 
the left and down. The fun thing is there is no detent between 1-2 slot and reverse. 
I made a block out mechanism at the gear stick which blocks you from pushing the gear 
stick to the far left. You lift the block-out mechanism to get it into reverse. 
This was also a time consuming task. The TCS 5-speed uses a mechanism on the back 
of the transaxle. Someone has it detailed on their website. 

This isn't a weekend exercise, but it is worth doing for a road car. I drive my car 
daily  to work, about 40km. Approx 30km on a freeway and 10km around town. The 5th gear 
is a delight, but could be taller. The R18 overdrive is 0.86. The TCS 5-speed is 0.80 
from memory.