I would go with the later rear cylinders that match the master cylinder. If not you risk the chance of having very sensitive rear brakes due to the extra piston movement in the smaller cylinders which could lead to a light feeling in the rear when the brakes are really applied hard. The other option is to install an adjustable brake proportioning valve so that you can tailor the rear brakes to suit your needs.
'73 Midget (V6)
'59 MGA (I6) under construction
'73 Lotus Europa
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