What are those websites? What are the dimensions? Are they dramatically different? There may be a deal of misrepresentation due to variations in thinking and designs of different manufacturers - GMB has the concave cups that mean to compress acting as springs once ujoint is installed. This actually is a measure against high frequency vibration - the cups act as springs "suspending" the shaft. The installed ujoint contracts in overall diameter, because the concavity contracts, taking a few thousands off the free diameter. Meyle which was used in my case, has flat cup bottoms - different idea...
I do not see why suddenly the ujoints have to be different between manual and automatic applications for cars of same era... It would simply not make sense to invest money and effort at designing a different system with a ujoint being a hair smaller in diameter... You're most likely worried too much. But let's see some details of which I asked above, perhaps I am wrong.