Type F fluid in 727

I think I'm just going to drain and flush with dextron and see if there is a noticable difference.
Another question, at wot what rpm should I be shifting at? I'll have to drive it and see where it is now and let you all know. It might be a few days as my duster is in need of my attenion right now.
Any ideas as to why I cant shift into 1 or 2 manually?