Dug up this site with old repair manuals. It's probably old news to you guys though.
http://chevy.oldcarmanualproject.com/military/1942/index.htm
Anyway, it doesn't identify the tranny but does give the ratios as follows:
1st 7.06
2nd 3.48
3rd 1.71
4th 1.00
Rev 6.98

I believe the later SM420 has a 7.02 granny