Earle,
The frame on a convertible is not the same as a sedan. It is "boxed" for reinforcement.
This site is a Buick convertible redo, but it will give you a pretty good gaze at what you are looking at:
http://members.shaw.ca/ritc1/again.html
Someone may know a better site that is more Olds specific, but an A-body is an A-body for a lot of things. And this guy really details everything pretty well.
Good luck and welcome!
C.J.