6BT sounds good to me, you could still use the Roadranger RTO-6610 if you wanted. Transfer case will be divorced if you need one.
I like Detroits, particularly the 53 series.
The simplest Detroit to fit and have decent power is a 4-53T. The biggest that could fit and still be usable is a 6V53T.
I have had both in different 1 ton trucks, the 4-53 is nice, the 6V53 is to big, and heavy for me.
71 series don't belong in a 1 ton truck without extensive work, would have an excellent chance of creating a boat anchor on overloaded tires..
For pictures of the 61 F350 with Twin turbo 6V53T and RTO-610, and my current project, a 48 Chevy 2 ton with 4-53T, Jake brakes, and RTO-6610:
http://community.webshots.com/user/Grigg3
Grigg