the 180b is probalby your best bet. 120y will be a series, skyline wil be an RB30 (6cyl).
you could put the bluebird motor in, but its probably a CA20, which isn't all that great. The 180b would probably be your best bet. Its virtually a straight swap (I think 521 is much the same as a 1600 with an L16 standard?) so the move up to an L18 is fairly straight forward, with a noticeable power gain.

brake wise i think the skyline is the way to go, although it will require a lot of effort. not entirely sure whats involved, but you will need to change the springs with some smaller items (possibly 180b or bluebird ones) and i'm not sure what kind of camber you will get, it could be postive (nasty) requireing strut tops etc. to sort out. I think there's something on this kind of conversion in the datnet forums.

Do a bit more research anyhow... I don't own (or ever worked on) any 1600's or anything like that.


[edit] whoops... just found out they have a series engines... in that case it'll take a fair bit more to get the L series in. How similar to 1600's are these cars? maybe you could get the crossmember out of a 1600??