Hi Chokudori!
The first thing we must clarify is that this car is a short wheel base model C110. Sorry if you already know this... but basically there were the long wheel base (in sedan and hardtop) and the short wheel base (in sedan, hardtop and wagon). The long wheel base models were powered by the L20, L20E, L24 or S20. The short wheel base models were powered by the G16 or G18 (at least that's all I have seen).
The extra length of the LWB is in the engine bay. This is, of course, to fit the six cylinder engine. Otherwise they are pretty similar. The LWB got IRS whereas the SWB did not. I would imagine they could be swapped over with a bit of work but don't quote me on that!
A six cyl L-series engine will NOT bolt up. This is the main reason for the SWB and LWB models.
Do not kid yourself - this was the base model Skyline and never had any performance intentions. If you really want a SWB (some people do) then I think it looks pretty good (prepare to find a fair bit of rust though from what we can see on the boot), but I would just buy the bigger brother GC110.
Like Warps said though, if you live in Australia, just buy a 240K (they're almost exactly the same to JDM GC110).
edit: here is a picture that shows the shorter bonnet length a bit better, for those interested