The expansion valve is located at the evaporator inlet, under dash close to firewall.
Sorry I don't have a diagram but I am looking for one. If you follow the small diameter a/c pressure line to the evaporator, you'll find it at the evaporator core.
You can get shop manuals here: http://www.faxonautoliterature.com/1967-Oldsmobile-CD-ROM-Repair-Shop-Manual-Body-Manual-Parts-Book-P13479.aspx
follow the line from receiver/drier to evaporator.
I found a link that will help you:
This link has diagrams.
Ok, I found a diagram and you should be able to access the expansion valve on engine side at the evaporator box.
The link HEATER AND AIR CONDITIONER - The Old Car Manual Project has a lot of information in it. Don't forget to rate my answer.
I'm switching to Q&A format in case you have more questions.