I've been up against much the same problem, as I wanted to build an A/FX version of the F-85 2-door, and got bogged down in the chassis research. A/FX allowed a lot of leeway in car-building, but within fairly strict limits which I wanted to adhere to.
The Johan chassis at first glance looks like a separate frame job, but it COULD have thin-gage sheetmetal hat-section rails spot-welded to the floor stampings and still be technically a unibody.
This is where my research stopped, as I was looking for period crash-books that would show the structure in detail, and put the project on hold before I found them.
EDIT: From 442.com... "Production began with the 1961 model year. The original Cutlass was built on a unibody platform. Rather small and low to the ground compared to the 1964 and later models."
I also found a scan of the '62 Olds chassis dimensions, and it is indeed a unibody, with hat-section sheetmetal rails, and a heavier-gage steel front subframe to carry the engine and suspension loads. The underbody appearance of the Johan kit is more-or-less correct, as far as it goes. There's enough there to make a basis for a nice one, with some work.

Edited by Ace-Garageguy, 07 December 2012 - 04:54 AM.