Springs.

[Big Vern]

Hi TT.
The length you need depends on the open/closed length of the dampers your using. So if you're using 'book length' dampers then you need 'book length' springs or you'll get into trouble with wrong bump and rebound lengths. Ash G. or any of the other who have 'finished' can advise on dampers used as can Saturn.
As for the spring rates you need to do as Mr H. says and find the corner weights first. It would be useful to measure the unsprung weights before your final assembly.
There are several 'programs' on t'internet you can feed this info into to calculate required spring rates. ( I use an 'industry' program but it's not user friendly so I don't offer it out as many proffesionals get tied in knots using it!)

Twin - on what basis would you try to fit 18" coil overs on a car originally designed for 14" open length? Spring lengths are likely to be 8" to 11" at most.....

Regards BV.