Hi Swordmd.
Your open cab US6 is the only one in existence that I am aware of and I've been interested and/ or involved in these trucks for thirty years.
There is a faked up one in the UK that was made using a CCKW cab spliced onto a Stude scuttle and a fellow here in Australia started putting one together the same way but the project lapsed.
As I recall the guys said they were pleasantly surprised that the GMC cab sat straight onto the Stude mounts. This plus looking at what photo's of the open cab US6 that we could find led us to believe that the cab rear of the dash were the same.
This may be a rude question but, are you sure that your open cab truck is the real deal? I would be very interested to know of any of these trucks that do exist, even the most miserable remnants, for to my knowledge none survived or only pilot models were built.
The open cab Stude is the holy grail of US6's.
David