The splines changed at the very late series V, the size of the clutch plate changed during the series IV when the first gear became a synchonized one and at the same time the clutch press plate changed from springs to diafragma.
Not disputing that at all, just trying to point out a few other considerations when doing this transplant. According to contempory literature there were small ratio changes between the Series IV and V gearboxes along with the clutch plate size. So perhaps prudent to check if the splines are the same?
There are more differences like for instance an alternator instead of a dynamo and the carburettor plus manifolds differ but the question was: will it fit directly and the answer is yes.
I was wondering if a series V 1725cc engine is a direct fit into a series IV 1600cc car, whether the 1725 uses the 1600 gearbox and what are the rear axles ratios for a non od 1600, and OD 1600 and an OD 1725? A lot of questions - thanks if you can help!