Here's why:
* Dashboard and Instrument Panel: The shapes, sizes, and mounting locations of the dashboards are completely different. The 1993 dash won't simply bolt into the 1986's frame.
* Wiring Harness: The electrical systems are different. The 1993's wiring harness won't be compatible with the 1986's. You'd need extensive rewiring.
* Door Panels and Seats: While some parts *might* be similar, the mounting points will likely be different, requiring modifications to both the van and the interior components.
* HVAC System: The heating and air conditioning systems are not interchangeable.
* Center Console and Floor Consoles: These will not be compatible.
In short, while you might be able to salvage *some* individual parts (like certain switches or trim pieces), a complete interior swap is impractical and would require extensive fabrication and custom work. It would likely be far easier and more cost-effective to either restore the 1986's interior or source a used interior from another 1986 G20.