Does anyone know what modifications will be needed when changing over a stock Turbo 350 trans to a Turbo 400 ? Do I need to shorten/lengthen the driveshaft ? I have a 69' Coupe

The driveshaft will have to be modified but I'm not sure if that depends on whether you have the short or long tail TH400, and the cross member will need to be changed. I think you may have to change the yoke also.

Don't forget the electric kickdown, the th350 uses a cable. You'll need the switch bracket and modified carb linkage, wiring etc.

Driveshaft, kickdown, and transmission crossmember all need to be changed.