Look for 'clean' sections of the casing etc and trace that clean section back to the relevant seal or gasket. What happens, usually, is that a slow seeping oil leak will keep 'washing clean' the area where it is leaking out from. Usually the dirtier sludge like sections are where the oil settles and picks up dirt, so it's where the cleaner sections are that oil is actually seeping.

If I had to make a guess, if the source isn't easily detectable, that it's the front pump seal where the convertor inserts into the front of the transmission. It's fairly common that eventually this seal will start to leak, just due to the nature of what/where it has to seal and how they spin and even a little bit to do with alignment of the transmission to the block, the 'centreline' of the crank and transmission axis of rotation can be a thou or two off, esp if the trans and engine were originally not mated together from the factory. But even then it's still possible with the original. TO be fair, datsuns in general are well machined and they don't tend to have alignment issues like that, so it's (imho) more likely that if in fact the front pump seal is leaking, it'll just be due to natural wear and tear, not misalignment, I'm just mentioning it to try and cover all bases.