You will need to confirm that the perches are setup to acommodate a 2.5" spring diameter. If not, you will need a different perch, such as the Eibach one.
Ground Control also sells perches. You could purchase their perch and use the spring that you already have. If you go this route, contact reflexgti for best pricing and let him know I sent you his way.
He will also be able to give you an estimate as to what the height will look like with the 8in spring in combination with the GC perch.
This would likely be the cheapest route for new parts.