Odometer will be significantly off, same for the speedo (and no, no replacement drive gear for the send is available for the W58 as far as I can find). You'll also lose acceleration, but your highway mileage will increase. Depending on how you drive in the city you mileage may suffer.
I ran a 3.91 from a pre89 GTE on my N/A for a while. I liked it for a commuter but I could definitely tell the difference when I opened it up.
If you're just trying to keep it moving, go for it, but it's not a performance upgrade.... If you just want a LSD then find a 4.30 LSD from a N/A.
__________________
If something breaks or you need to contact a member of the administration please post HERE. Unless it's a private or administration matter please post it on the forum. It benefits no one else if car related questions aren't posted for future users and takes away from the time I'm able to spend helping on the rest of the forum.
If you're so inclined I'm always more than happy to accept tips via PayPal. 
Tip Jar ---> 
|