I am wondering what advantage this swap has to offer.

The brakes [at least in Australian models] are the same as the 1200, & the axle splines are the same, so even if the axles thenselves are larger, this area would still be a weak link in the chain if a high output engine is used.
The B210's differential, [if it is an H150], has a 3.9 to 1 ratio in most, if not all B210 models which is the same as the original 1200's ratio, & this slightly bigger/ stronger H150 differential can be swapped into the original H145 rear axle housing with just a spacer & two gaskets.
Other ratio's available in the H150 range include both 3.7 & 3.54 to 1 if higher gearing is sought after.

There was a Borg Warner rear axle assembly used in locally manufactured B210 models [sedans] in Australia from July '76 & this is often touted as being superior in some way, but it was not used in US models, [which is where I am guessing you are at] or anywhere else for that matter, so was not included in the comments above.

Thanks

Yes, some 120Ys have rear coil suspension: those of the B310 type (model years 1979-1982). The earlier B210 120ys have leaf spring. Yes it can be made to fit, see What Diff and LSD should I use?.

It seems that the 120Y has coil springs, I think the 1200 2 door has leaf springs. The ratio would probably be 3.7 or 3.9, it would have rear drums, so that shouldn't be a problem, but don't know about the driveshaft or e-brake.

Can I use the rear Axle ( dif, brakes, etc) of 120Y Station on my 1200 2doors?
Can any one tell me the final drive ratio of 120Y Station?
Thanks