There was also a 707ci. GMC 6 cyl. gas engine,first available in 1950.
While I was searching around, I did see something about a 707, but it was hard to believe.
I also read some interesting info about GMC back in the old days. There was some pretty impressive stuff. GM even won an award from the US Government for their contributions in WWII.
Read more here, along with some pretty good pictures:
http://wiki.gmnext.com/wiki/index.php/A_Brief_Outline_of_the_First_Century_of_GMC_Truck_HistoryBill