The full syncro box from the S3 is a easier box to change gears ( slowly ), but I prefer the S2a box for speed of changing a gear and it's strength as it a little more robust.

The front portion of the gearbox of a later six cylinder model 2a will swap over with the front of a S3 box, so If you want to fit the S3 clutch to your S2a gear box, that is possible.
The difference between the earlier S2a four cylinder gearbox to the later S2a six cylinder gearbox is the later has a larger end on the layshaft where the front bearing sits.

If your changing the rear rover style diff of the 2a to a Salisbury S3, you will need to swap both the diff housing and the shorter prop shaft as the end of the pinion on a Salisbury is closer towards the gearbox.
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