I replaced all the springs on my N/A Ex mil 90 with the same military spec ones, harder ride compared to standard Civvy ones and carried the weight.
Part No's.
Drivers side Rear NRC 9448 (BLUE & RED)
NRC 9462 (GREEN YELLOW & RED) Heavy Duty
Passenger Rear NRC 9449 (YELLOW & WHITE)
NRC 9463 (GREEN YELLOW & WHITE) Heavy Duty
Drivers side FRONT NRC 9446 (BLUE & GREEN) 2.5 NAD
Passenger FRONT NRC 9447 (BLUE & YELLOW) 2.5 NAD
These are straight from a Army parts list so may have been superseeded knowing Landrover but at least it gives you a starting point, hope it helps.
Also found this which might help, I cant open it at work but should be spring rates
http://www.ep90.com/assets/docs/SpringRateInfo.zip