First I'd like to know why you think that a Koni shock won't fit on a 2,5" lift?
The front shocks are all the same size. The lift comes from the thoughness of the spring. (that and I am now running a 2" lift with al stock shocks!)
* To avoid shocks to overcompress, do I need rear and front bumpstops ? Which height ? Can you give me links on web shops to get them right in France.
Yes you do need bumpstops. You can make the rears yourself from a rubber hammer or hockypucks. For the front I ordered 3" Terraflex from Trailquest.com in the US. Shipping was $28 to Barcelona.
http://www.trailquest.com/teraflex/KJ/suspension.shtml#bumpstop
* Do I need to change A-upper arms ?
No you don't, not with a 2,5"lift.
* As the front still looks lower than the rear, can i make a 3/8 clevis lift without any issues on the strongness ? If yes, where can i order for conduit nuts on the web ? Or is it better to put upper plates ? Or both ?
you can level it, just do a 10mm clevis lift. You do not need conduit nuts. No need for a top plate either.
* Except OME shocks (that we all know), Koni shocks (that don't fit a 2.5" lift) and Proflex shocks (European racing ones for 1000 $ each Shocked ), what are the strongest shocks you know in US ?
I am not in the US but for what it's worth:
OME
RANCHO
KONI
Unless you are talking about the rear axle, then there's the OME
132L or Rancho 17004. Those are aprox. 2,5" longer then stock.
You can get those in the US. Yeah I know, shipping is high but you´re still cheaper then buying stuff in Europe.