Will a 6cyl 280se W126 auto box fit V8 W126 380?
 

does anyone know if the auto boxes are the same on 6 cyl and v8 w126's? or will it fit behind the v8 3.8.

No, they have different bellhousings

can i swap bell housings?

You would need depth guage & shims to set the axial bearing clearance on the K1 clutch drum...also the kickdown cable would have to be swapped...the driveshaft output flange....OF course the trans wouldn't shift correctly as the design of the valve body & governor is completely different for a 6 cyl VS a 8 cyl.