I really don't see the need for a 6-speed either, unless you plan on running a small displacement engine.
We also have a Superduty diesel with the 6- speed, and I skip gears in it all the time when not towing. 2'nd-4'th-6'th.
DV,
CR must have the engine set further forward in the chassis to have an 18" driveshaft.
My driveshaft is just under 9" with a TKO. My water pump pulley is several inches behind the front wheels. Maybe that's how I managed a nearly 55% rear weight bias.
Later,