Andreas,
235x75R 15's should be no major problem.
You have mentioned 1 of the 2 easy ways for a lift,
longer spring shackles, which can be made out of 3/8 " Bar Stock.
The other is if there are "Donor Samurai's" in your area, add one leaf to each spring set,
use the first leaf below the main leaf, or if you have access to a cut off saw/tool,
use the main leaf and cut the eyes off both ends. This is going to stiffen up the ride a bit,
But the big advantage is, doing this will help stop the main leafs from bending
just behind the spring Eye, while off roading.