Your stock axle is going to be about 62.5" WMS-WMS(Wheel Mounting Surface). I don't know much about the Dana swap, though. I'm going to try to steer you into the 14-bolt full float direction. Most of your dually equipped GM trucks from 73 on up have 63" WMS. I purchased one that was actually 62.75". You'll have to cut and relocate your spring perches and shock brackets if you go this route though. If you can't find a dually axle you can alway use an axle from a single wheel truck and use your Eaton hubs. The Eaton hubs will give around a 65" WMS. You'll have to get creative with your brakes though. Fortunately GM has probably a dozen different options for the 14 bolt.
