With the tail shaft you need to have one made with the L series and a series shaft ends, then it gets balanced. (not a big prob) the clutch petal its best to convert to hydralic as per L series gearbox,but you can re use the 120 petal no probs. With the standard diff i think your asking for trouble there. But see what happens.

i think the conversion idea would be good because its still a datsun motor and different to most other 120Y's. The L series can be a serious engine with a bit of work or a turbo.
I think you should go for a L20. all the 180B stuff is the same and the more power you have to start with the better.