Im considering doing a L series convertion in my 120y, but before i start buying all the bits for it i just want to ask a few questions.
From what i have found by doing a search i need.
Engine / Crossmember = L18/180B
Gearbox / Crossmember = L series gearbox / 180B crossmember
Brakes ect = complete 180B front end i.e. struts, LCA, tie rods, sway bay ect...
here are the things in not sure about.
Clutch Peddle = can i use the original 120y clutch peddle or will i need a 180B peddle box?
Tail Shaft???? = not sure which one ill need.
Diff = i plan on running the stock 120y diff because im not planing on driving it too hard

If there is anything i have missed or that i will need to know please tell me.

Plus i am going to be buying a 180B for all the bits and peices

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.

the BW 120Y diff will handle a stock L-series no trouble. h145/150 might be a different story.

Get the clutch master and slave cylinder out of the 180B as well.

Use an early 80s bluebird tailshaft. (or if the 180b is a one piece use that)
Stick with the 120y swaybar (you will need to trim a section out of the 180b crossmember).
Use the 180b radiator.
The diff may or maynot cope.
The speedo will be out a bit.
You will need a 90 degree bend on the speedo cable .
You may have trouble closing the bonnet, cause the standard carb and air filter may hit.
I went for a pair of side draughts, but that is probably not the only way to overcome the problem.
Cheers and good luck!

120y is probably the easiest car ive ever seen to convert to hydro clutch, studs are there to suit a 180b clutch master, hole is already in the pedal to suit all you have to do is get a hole saw and cut a hole

180b tailshaft is to short, if you get a stanza sss gearbox or an r30 stumpy then you would most likely be able to use an auto tailshaft to suit the 120y BW

but tailshaft isnt really a hard thing to work out once the engines in

so here's a question I've been thinking about for a while, would the 180B crossmember need to be notched\ modded for a L20 motor?

for L20 motor, you either reverse the x-member as was commonly done by the 510 crowd, or swap sumps on the motor which also means you need to make the dipstick fit the new sump position.

Use the short/stumpy boxes - Stanza and MR30 Skyline and you shouldn't have any problem with speedo drives

oh and my early 120Y (1974) does not have a hole in the clutch pedal, not that that's a big deal.

I remember hearing vaugely that the L18 has a much wider/better torque curve than the L20B - never really got into L series yet....I bet the Ozdat/datnet guys would no a lot better.

The L series motors are great, i have had both twin webber and turbo charged L series and they don't dissapoint.
Even thoe at the moment i have an A14, i still miss the L series and would concider the L20 as my next 1200 project motor after a SR or FJ20.
See what the engineers and my wallet say!