They where used in the first dodge cummins automatic trucks, they hadn't come out with a 4 speed auto that would hold up to the cummins so they had dana build odd ball 60's with 3.07 gears to be able to run a 3 speed auto behind the cummins.
All that needs to be done is buy a 4.10 carrier (dana part # wc-706400x) for the 61 and run a ring gear spacer which will allow ya to run any regular 60 gears.