Hey Marc,
nice sedan very neat, if you are chasing some new tailights for it i have a pair that are in pretty darn good nick, and also custom door cards and parcel shelf. the body is going and the interior bits were going with it but if your interested i can dig it all out for oyu, and anything else you might want. panels are shot but trim is good, and so is the chrome.
As for the L series, your easiest bet is to get the following from a 180B:
180B front end minus Struts
Radiator
clutch master cylinder
then get a stumpy 5 speed from a r30 skyline or a stanza. if not any 5 speed will be fine you will just need to shorten the tailshaft. if you get the stumpy the stock item will fit.
then drop the whole front end out of your car, replace all your bushes in the 180B gear cos it's out of the car, tie rod ends and all the rest. then bolt the 180B crossmember into the Y, the 120Y's have 3 crossmember bolt holes on either side of the engine bay as they were released overseas with L series as a limited run. use the 2 bolt holes furthest from the firewall, and then use the 120Y gearbox crossmember for the L series gearbox. it all bolts up without needing to cut anything.
then below where your clutch cable comes out is 2 studs on the firewall and between them is a pressed circular indent, get a holesaw the same size as the indent clutch master bit, cut the hole and slide in the master, then mark on your clutch pedal where the linkage bracket is take the pedal out and drill a hole for the bolt to pass through bolt it back up, clutch done.
you can use the 120Y struts on the 180B LCA's or go for a bigger brake package.
at the end of the day if you can pick up a complete clapped out 180B or even one that is missing panels you can do the swap.
any more details let em know. i'll get some photos if you want any too.
Cheers,
Team_Dat