I'd like to hang a 5-speed AT behind a 603 but I won't do it if there is heavy electronics work involved. Ideally I'd use a 722.6 but I understand those have a lot of electronics.

Shifting from 3 to 5 isn't that big of an issue if it's not too far a drop. 5 speeds is nice but overdrive is key.

This assumes there is enough similarity between a 603 and 104 that a 722.5 will even bolt up.

Sixto
87 300D