Shorter wider springs are for the front, longer thinner ones are for the back
Although front and back is usually printed somewhere on the springs
As for lowering, general concencis is that you're usually safe to go upto approximately 40mm lower on the standard shocks before you need to start looking into shorter and stiffer shocks.
This however has a tendancy to vary a bit between different suspension companies due to the real world increase in spring tension and tighter/shorter spring loops.
Cheaper springs (as I found in the past with Jamex springs on my old Bravo HGT) proved to be generally crap and bumpy both on standard and slightly uprated shocks.
Eibach however have always had good customer feedback and they seem very dedicated to building springs that work very well with the standard set up around them. So if they say that you do not need to uprate the shocks when fitting their springs then you can honestly believe that you do not need to uprate your shocks (I know their Pro kit is as safe as the standard springs are when coupled with the standard shocks).
Because of this you shouldnt have any worries about just uprating the springs, the shocks will be fine and you wont be bottoming out the springs (well no easier then you would bottom out the standard springs which I'm yet to see the GPS do).
As for fitting, it's relatively easy as long as you can get your hands on a hydrolic jack, set of axle stands, decent socket set/ratchet spanners, breaker bar, torque wrench (optional), spring compressors (about £10 from motorfactors) and someone to offer a hand.
The rears are the easiest because you dont have to compress the springs, the basic proceedure is:
- Crack the wheel nuts
- Jack up car and remove the wheel
- Put car on axle stands
- Use jack to take compression off the rear swing arm while undoing bottom shock bolt
- Carefully lower jack and pull swing arm down to release the spring
- Pop new spring in and turn until it's in position
- Carefully jack the swing arm up again until spring is firmly seated
- Pop bolt back through bottom of the shock and tighten
- Lower jack and check that the spring feels seated
- Pop wheel back on and tighen all bolts
That's one side done, move on to the next side.
The fronts are slightly more complex due to having to compress the spring to remove it from the shock. The basic steps are:
- Crack wheel bolts, jack car up, put on axle stands and remove both front wheels
- Crack the tight lock off the central top bolt on the shock before going any further (this is a PITA to do once you've removed the shock)
- Remove the 2 (17mm I think) bottom shock bolts from the hub carrier
- Remove the 3 (13mm I think) top shock bolts from under the bonnet on the top mounts
- Unclip any brake lines that are clipped to the bottom of the shocks
- Grab you mate and get him/her to help you manouver the whole unit out the bottom of the wheel arch
...... now the shock is out:
- Attach the 2 spring compressors to each side of the spring and carefully tighten in turn (half a turn at a time) until the spring is no longer compressed against the top mount
- Now carefully remove the top mount bolt (that you sensibly loosened off before removing the shock)
- Once the top mount is off, remove the spring and then untighten the compressors in turn until the spring is free
- Remove the rubber spring locater from the top of the spring (usually twists on)
- Refit the rubber spring locator to the new spring (beware that this usually moves slightly when compressing the spring)
- Attach the spring compressors to the new lowering spring and follow the same proceedure above
- Once compressed put the spring onto the shock and line up the bottom of the spring with the seat on the shock
- Put the top mount back on in order and tighten the top mount bolt
- Once the top mount is secure undo the spring compressors as above rotating the top mount to line up as you go
- Check the top mount is tight and the spring is in place before refitting
In true Haynes style, refitting is the reversal of removal
If any of that doesnt make sense or if you get stuck at all let me know/drop me a PM and I'll try to clear things up for you