They are two different beasts a 400 and a Deauville. A Longchamp and a 400 would make a better comparison, dimensionally and handling that is.
As for converting to a manual. I think that you need a second series subframe. This placed the engine 4 inches further back and used a better responding ZF steering rack.
The frame and rack were used on Maserati QP 3 which shares the platform with the later serie Deauville. So if you can find a manual QP 3 for parts than you are good to go!
I would stick to the autobox. It suits the car.
Chiel
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.