Joe, congrats on the electrical upgrade. It was the best thing I ever did for my '55.
I'm not familiar with DeSoto's, but here is some insight from another board:
http://www.forwardlook.net/forums/forums/thread-view.asp?tid=7151&posts=7 "A 6V starter will work on 12V no problem, the 6V wiring is actually heavier than the 12V wiring is. ...as long as the tooth count on the gears is the same, they will interchange. You could even use the complete '55 starter on the '56 motor without any problems."
"According to my '55-56 MoPar parts catalog, '55 and '56 are interchangeable BUT only the 6 cylinder starter with a 6 and 8 cylinder with an 8. Make sense? The Bendix drive is MoPar part number 1473219 for the 6 cylinder, the clutch (drive) is part number 1343708 for the 8 cylinder. Judging by the part number for the 8, it is probably the same as a '54 8 cylinder."
That's my experience, too. I am still running (for the past four years) on my 6-volt starter. I did change the solenoid to 12-volt, but mine bolts to the fender apron, and can be found at any auto parts store.