Anyone who is building a 540 who wants coil springs in the rear that are adjustable I found a company in Sydney, Wholesale Suspension, that can supply them. The springs are made for them by King Springs and are rated at 380lbs.
They supply the adjustment pillows and springs. They will all supply adjustable length adjustable dampening shocks, but shorten shocks will be okay as long as the spring doesn't fall out when the wheel is at full travel.
Warren