What i want to achieve is to raise the terrain generation level for about 100 blocks, so i have a deeper world instead of the 65 blocks now.

I also don't want to change much, existing terrain gens from minecraft and other mods should just continue to do what they are meant to do, with the difference that there is like 100 blocks more underneath it.

The reason why is. I want to create layers of stone (3-4) and a height of 65 isn't satisfying enough.

Minecraft world height is limited to 256 and many mods already use all height to generate high mountains. If you don't worry about not supporting most biome and world gen mods, that you can proceed. Otherwise, it's not possible (well, it is. But raising height limit above 256 is a pain, nearly impossible with forge, and will make every mod incompatible with yours).

So, to raise height by 100 blocks during world gen, you should create custom ChunkProvider, that contains parent chunk provider (which would be used by default for world gen). For all block gen methods, you call parent.method and then raise all generated blocks by 100 and then add yours. Now when world is created you have to get current world's chunk provider, create you provider with this provider being parent and then set world's chunk provider to yours. You will probably need to use reflection.

Also, make sure that you're doing this in correct dimension .

35 blocks isn't going to make that much difference to the giant 191 block tall mountains.  Oh no, 156 block tall mountains!  They're still 93 blocks taller than the vanilla extreme hills, which, to be frank, already feel huge when you're looking at them.

35 blocks isn't going to make that much difference to the giant 191 block tall mountains.  Oh no, 156 block tall mountains!  They're still 93 blocks taller than the vanilla extreme hills, which, to be frank, already feel huge when you're looking at them.

I do not understand what you are trying to tell me or perhaps you were talking to Elix ?

Where you get that number 35 from? 65 + 100 = 165 i think 

I don't want to make mountains bigger (not at all), nor do i want to go above the 256 blocks.

If you know the mod "terrafirmacraft", that's a good example.

But nevermind, i think it would be a bad idea anyway.

I misread, I read "to 100 blocks" to "by 100 blocks"