I dont know what year your F650 is, but the 6b is an option for the latter ones. It is tight, but I know they have been fit into more than one Ford body. Piers Harry (PDR founder) has a 78 F250 or 350 SRW with a twin turbo 6B.

DP

 

I have a 78 F250 with a 6bt and it was tight. With the heavy radiator there is about 1 or 2 inches of clearance with the radiator. The Firewall has a decent amout though. The truck also has a 3 inch body lift installed which helped quite a bit with clearance issues. Visit www.fordcummins.com if you haven't already. They provided me with a bunch of good info while doing my swap. Good luck! Best thing I ever did for the old truck...