IRS spring weight

[Bushbasher]

Does anyone know the spring rate of the factory AU IRS rear springs? From everyone I've spoken to it seems the same springs were used in the SWB cars as the LWB Fairlane which would explain why on the Fairlane the rear springs are marginal at best considering the extra weight of the Fairlane over say an IRS Fairmont.
My problem is, I just replaced the rears on my 'lane to something that was described as a stock height, 25% over stock rate HD spring suitable for LPG and/or towing only to find my 'lane is now stiff as a board and 50mm higher than stock. When I called the manufaturer (Industrial Springs in Adelaide, marketed as Aurora springs) I was told the spring rate was 720lb . So clearly they have to come out but what to put in hmmmm??
After calling King springs, Lovells and Pedders looking for info on standard height springs to suit my need both Kings and Lovells were happy to give me their spring rate on their respective recommended spring and both said "about 20% stiffer than stock although both gave me different figures. Lovells on the other hand were very cagey and wouldn't tell me anything but tried to convince me that their recommended spring was the go and I wouldn't do better. :
I spoke to another bloke at Industrial Springs, and after I measured the standard spring, he used a formula he said was the standard way of gauging a springs rate that gave a figure of about 230lb for the stock spring; but on that number the "20% stiffer" quoted by Lovells (@400lb) and Kings (@435lb) are way out. Industrial Springs said they have an alternative @ 330lb that on the face of it sounds close to what I'm looking for ( if 230lb is correct for stock), but unless I KNOW the stock spring rate to start with I don't know if it's going to be any good.
What I'm aiming for is countering the extra weight of the gas tank but retaining as close as possable the ride quality and balance I had to match the front end which I'm quite happy with.

So to summorise

: If 230lb stock is correct then 330lb may do the job.
: If 400-435lb is 20% as Kings and lovells claim then 330lb is closer to stock so no good.

So I really need to know the Stock spring rate now before I move again and nobody seems to know, not even Ford have been able to tell me, so back to my first question; Does anyone know the spring rate of the factory AU IRS rear springs?

Any info here would be most gratefully appreciated .

Cheers
Bushbasher