250lb is 4.5kg. Too stiff imo. But it depends alot on what the wheel rate works out to be, which depends on how far inboard your lower coilover mount is. Assuming its in the standard location for the shock, I say that's far too stiff for a street car.
I don't know where dattodude got that figure from, but it's either a typo or a plain old mistake. Kings lowered leaf springs for 1200 are 150lb/in.
What's soft for a race car on slicks or semis can be hard for a street car on normal tyres. The more grip you have the stiffer the spring rates you need.
Unless you have 50:50 weight distribution, I wouldn't be going for the same spring rates front and rear if the rear is anything like 1:1 motion ratio - it will make it very tail happy!
This page from Pitroad shows the spring rates they use on the heavier B310 4 link rear end.
http://translate.google.com.au/translate?sl=auto&tl=en&prev=_t&hl=en&ie=UTF-8&eotf=1&u=http://www.pitroad-ts.com/A-parts-2/bushing/spring-rr310.htm&act=urlThe Pitroad spring 3kg is what they use in their TS Cup race car on 9" wide rear slicks. That's 165lb/in. They cut 2 coils out of it for the race car which will make it slightly higher rate - say about 175lb/in at a rough guess.
The normal spring 1.7kg is the stock spring rate, which is 95lb.
NISMO 2.26kg is 125lb.
The best bet for a custom setup is to go to a race type suspension specialist, get the car scaled and work out the natural frequency as a starting point. Anything else is just taking a shot in the dark. But if that's the approach you want to take, I'd start around 150-175lb assuming the spring location is pretty much where the leaf spring was.
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