what is needed to be done to the rear springs in the 1200 to make them firm enough to drive fast around corners.

i know that you can add another spring, do you just use another set of 1200 springs and take the main one and add it to the rest of the set.

also whats the best way to go when lowering them, to use lowering blocks or to get the eyelets reversed, and what does this normally set you back $$$.

if there are any other options for firming the back end up please let me know.

Get some rear leaf springs out of a 120Y and remove the main spring and pu the main spring of the 1200 on to the 120Y ones. You now have an extra leaf.
Some people haven't liked this though as it make sthe ride to hard (Or something)
I haven't actually done this. So I don't know how it works out.
Have you checked out Martin's Datun 1200 page?
http://members.iinet.net.au/~mpot/DatsunMods/

Lots of goods to get you started off...

1200 rear spring modification is another highly subjective topic. Some people do fine with stock springs and lowering blocks. Other guys swear by reverse eyelet type with custom, firmer leafs. Then there's the 120y conversion (see above). If you're going to be drag racing or running high horsepower, you may want to install panhard rods to inhibit side-to-side movement.

panhard rods thread.

Has anyone gone through the trouble of converting to IRS?

i did 120y thing, it was no good, it lifted the rear causing understeer and was probably too hard

i found new shocks and urethane rubbers (or any non-30 year old rubbers) made a huge diffrence, lowering blocks are good too, i havent had reset leafs

The right shocks can TRANSFORM the handling of an otherwise crook car. I use Koni's for road duty. The adjustment can be used to compensate for wear, & also to tune the suspension for your useage on the day.
Chris