He is probably right, he built it. My first attempt was to lengthen both sides, I had a spare set. After installing the drivers side, I noticed I had to push the diff backwards to get the bolt holes to line up. (Without the drive line in it can be done.) Then brain engaged, and told me the mount hangs at a 45 degree angle, which is why I was shoving it back. I cut it again at 45 degrees and slid it down at least 1 1/2 inches. It then seemed to hang more naturally. When I built my steel diff housing, I made both mounting brackets 1 1/2 inches longer than stock mounting spot (sitting higher on axle), enabling me to use stock hangers.
Take an angle guage and slide under your rig and get the angle it hangs (or use paper and approximate the angle). draw it on paper and mark the position it is and where you want it (how much you want it to drop). Make sure the two of the three bolt holes are lined up and down. Now you have a template of how it should look.
Are you going to rotate the back side upwards, or drop the back crossmember down? If you just allow the back to rotate up, the new mount needs to be turned also. (Always more things to consider.) You might open a thread on the forum about the possibilities of turning the third member over and running with the input shaft on top. That way when lowered it will not run into the crossmember. It may start turning w/o oil, but the gears should move the oil up immediately. Maybe someone has done it, or knows why not.
Let me know what you do.
Steve