I saw a video on YouTube of 1956 f600 fire truck with a Cummins conversion and a dual transmission setup so I know it's possible. I'll post on this thread as I progress, but any help I could get would be more than appreciated.
The 56 Cummins conversion you mentioned is mine, and I also recommend you get a donor truck ... I paid 2k for a 94 f700 .. all I used was the engine, fuel tank, batteries, fan shroud, tires.
56 f600 5.9 cummins 6+4