OK
If you now have the 3:69, then I do not believe the 3:27 gears from a 4.5 will fit in your differential case ..the case is larger on the 4.5. to accommodate the larger ring/pinion........
That is why I mentioned the extra oil cooling return tube..if your 3.5 case has the tube, you have the larger case..if it does not, then it is the smaller case used for the 4:08, 3;92. and 3:75 gear sets. If that is the situation, then going to the 3:27 [ which I recommend] will require the whole rear end change out rather than just the gear set.
A note**
If you find the 4.5, be aware that many of them have a problem with the casing rounding out where the spider holding pin goes through the case. This is common on the 4.5 due to the engine torque and car weight. Check that before you buy.
After a diff change, you want to center align the axle to the chassis..I have
designed a homemade measure jig/tool using simple copper tubing fabrication that you can make for about $5-$10 in parts and fittings that makes centering the rear pivot pin to factory specs real simple..if you want that, I will post a jpg .
[ I think they even have a copy of that in the archieves over at 113 group somewhere, might even be one here ..try the Search Feature]
Here is a video from the 113 site that shows that yoke/pin failure complaint. These 3:27 are also a preferred mod on the 113 , but in that case , they plug the oil cooler/return line I mention due to lack of body clearance on the smaller SL chassis..[ they just eliminate the tube with plugs]
http://video.google.com/videoplay?docid=6365474313884505435&hl=en