"Tybalt", thank you for your comprehensive comments.
My forum post was bought about by my desire to obtain a much "softer" and adjustable ride than the factory spec of around 310 lb/in.
"Performance" springs are a no go for me. Ok for a track car.
The factory springs are about 11" free length.
Thoughts were to use 3" ID springs on the Mustang C/O shocks that can be bought outside QA-1, eg Eibach, perhaps even modifying the TR upper spring tower and/or the bottom spring plate to overcome using springs that may be too short.
My other options are to fit ordinary 250lb springs of suitable length, wire diameter, number of turns etc that would probably have to be especially made, or better still have variable rate springs custom made.
I remember someone on a TR forum many years ago, perhaps in Germany? offering to make variable rate springs.