Looked at the 56 Service manual today. The wheel base on the Special/ Century ( 40/60 series) is 122" for 56. The rear tire tread width is 59". The wheelbase length on the 50/70 series is 127".
I cannot find the measurement of the length of the driveshaft or torque tube but the book shows only one regardless of the model. And the book does not have different proceedures for the rear axle repair between the 40/60 series and the 50/70 series BUT the rear tire tread on the 50/70 series is 62.2" so obviously the rear axles are different.
Again, you have to account for shock absorbers to mount the 56 rear axle in your 53.
Good luck
_________________________
John C. De Fiore BCA # 3757
56 Super 56R
69 GS 400 Convertible
69 Electra Limited 2 dr.( well, no longer limited although still unique)
95 Riviera Supercharged (sold)
06 Lacrosse CXL
Doing my part to stimulate the economy!
Member of the UPSTATE NY CHAPTER
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