Hey
Whats the best way to upgrade a 1200 sedan rear suspension? is best to use 120y leafs suspension and replace the 1200 sedan leafs.
I have also heard you can reverse or flip the orginal sedan leaf and that is better? Is this true?

Mate, this subject has been done to death.
Try looking on the search bar on the LH side....

The general consensus seems to be that the best option is JT racing springs... Around $220.

There's varied opinions floating around on flipping leaves & using lowering blocks etc.

Not too sure on the 120Y option.......

I'm running a ute with standard springs & lowering blocks & that seems to work o.k.

get them reset and stiffened with another leaf, fit new shocks and new bushes (prefferably nolathane). Thats all you really need on a street car

you can't replace the 1200 leaf springs with 120Y leaf springs - they're too long.

King Springs list a replacement leaf for 1200s. Fulcrum in Brisbane have a listing as well.

i had 120y springs in the back of my sedan and i know of a few others with them too
the john taylor springs are also around 330 from memory but they are 2 in lower than standard and also stop axle tramp

I've also seen it done (120Y oupe leaves installed in 1200 coupe) and the rear shackle reversed and jammed due to the main leaf being too long.