74 Duster, resto mod 6.4 conversion in Australia

By sliding sections of shaft do you mean you essentially making the shaft longer so no adapters required? Do you have a link to the items?

The stock coupler is built so the steering shaft moves inside the coupler some as the chassis flexes. If you remove that, there is no give and the shaft has to collapse or extend instead and it isn't intended to do that.

Steering shaft upgrade