Ok here's my thoughts. The B series engine have a lower operateing rpm than the 440, therefore your going to need a overdrive somewhere in the drivetrain to be able to get down the road at a decent speed. Also just because a trans does good behind a monster gas moter dosent mean it can take the low speed torque of a diesel. But there are good Dodge trans that are fitted behind the 6B and changing the trans out is a good idea. What you need to find out is the gear ratio on the axle and find out how this works with what you have and make adjustment if necessary.