I have a 3.77 english diff with a small flange and wanted to upgrade to a 3.54 diff ( for better cruising etc) however the ones I have seen have the larger flange. There is a single nut which holds the flange on but I have been told the crush bearing needs to be changed and special tools needed to get the correct tightness etc? Is this correct? or is this something which can be done with no special tools etc?

In theory you should fit a new collapsible spacer and use a proper gauge to check the resistance, but I've had reasonable luck with just retightening the new flange to around the same tension as the old one.