I parted out an original 68 cuda FB over 25 years ago. It was a /6 and had a 150 speedo. Sold the dash and interior parts at a swap meet cheap. Wish I still had them but I didn't know I'd have another 68 years later.
You mentioned in a previous post that you had not found the build sheet yet. My 69 'cuda 340 had the build sheet in the front bench seat cushion.
Maybe you found it already but I'm reading earliest to latest posts.
I'll be watching because my long term plans for my 68 are 5.7 and 5 speed manual.