I checked the GM Powertrain website. From that site, and what I know...
A 4T60-E used in a '98 Century will bolt up to a 3800 Series II.
The 4T65-e is used on the current 3800 Series III and also the 3900 (I think the 3900 is a 60 degree V6, and then the Series III is a Buick V6).
A Getreg 5 speed manual used in an early 90s Monte Carlo Z34 and other 3.4L DOHC W-body cars bolts up to a 3800 (shows the 60 degree tranny bolts to a Buick V6).
here'* a photo of a 4T65-E. I'm thinking this will work.
'89 Buick Reatta. Formerly propelled by a supercharged LN3, soon to have Series II L67/ 6 speed manual http://<a href="http://www.bonnevill...sc&start=0</a>
'93 LeSabre Limited...$450 fixer upper my friend bought and is having me fix up though currently my driver for the time being