A 6.3 axle is not a direct replacement-it does not accommodate the rear compensator/spring and it has attachments for the anti-dive which need mounting points on the body. Also the ratio is too low for a 4.5-it will be quite sluggish.

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Tony H
W111 280SE 3.5 Coupe
Manual transmission

Past cars:
Porsche 914 2.0
'64 Jaguar XKE Roadster
'57 Oval Window VW
'71 Toyota Hilux Pickup Truck-Dad bought new
'73 Toyota Celica GT