The nv3500 in my regency does not like to shift without being forced into gear at anywhere over 4k rpm. It has had a pretty rough life and is a fragile transmission from what I have heard. So i'm looking to upgrade. Therefore does anyone run a tko 600 in theirs or just go straight to a t56? Also what has to be changed when going to either? The extra gear would help a lot because I do a lot of interstate driving. As for know it's just a stock truck but I'm looking to do some modifications as far as cam and possible supercharger. Just tryin to figure what'd be best for a transmission right now.