Most of the WSM's contain a wealth of info on front springs.
free length - static laden length = compressed distance >> static laden loading / compressed distance = lbs./inch design load rating
S III >> 11.65 - 7.85 = 3.8 ............... 950/3.8 = 250 lbs./inch
S IV >> 12.62 - 7.55 = 5.07 ............ 1040/5.07 = 205.13 lbs./inch
From these numbers, the presumption is that Rootes used a longer, softer spring to give a softer, more compliant ride quality in the S IV. The extra length is there to give the same approximate ride height - or a bit higher - to help prevent bottoming the suspension on bumps and negating the softer ride quality.
Another footnote to consider, the OE spring rates and shock valving was designed to accommodate bias-ply tires, not radials. Radials tend to be both more absorptive of road imperfections, due to the soft sidewall construction, and heaver than bias ply tires of the same size range - not even considering the larger upsized wheels and tires most want on their cars today. All of these changes should be supported by stiffer springs and shocks just to maintain parity with the OE design parameters. How much stiffer? Unknown, but a guess of 10>25% higher rate is probably on the low side considering what equipment most are saying they are installing these days.
Just some food for thought.
HTH
Don