Nut is finally off. I took the gearbox to my friend's shop. tried with the impact and got nowhere. He looked at the amount of play between the input and output shafts and thought there was too much for the impact to be effective. That made sense to me, what he did next I would have never have thought of. He placed the nut with the spline in it in his milling machine in the vice. We put the trans, bellhousing down in the mill and put a Tbar on the socket. Then he put 2' extensions on each end of the tbar, climbed into the mill and I pulled and he pushed. After a couple of really hard grunts it broke free. The milling machine was the extra mass we needed to stabilize the input shaft and finally get some force on the output shaft. Hope this helps someone else in he future. That was one tight nut!