[1.6.4] Generating blocks AFTER vanilla generation is complete

I need my mod to have the ability to generate blocks in a world after vanilla generation is complete. I am not using custom dimensions or biomes, and I don't want to change vanilla generation at all.

The mod simply adds new plants to Minecraft, some of which are two-blocks tall. (Plant Mega Pack)

PROBLEM: my generation code is not being called at the right time - I need it to be called last when each chunk is generated.

Because of this:

- Near rivers/lakes, when I place plants on dirt, Minecraft sometimes replaces the dirt with sand

- When I generate two-blocks tall plants in desert and jungle biomes, the bottom block is often replaced with an air block

- I cannot detect light level in a forest to plant mushrooms, because trees are generated after my plants are

There is a hook in BiomeDecorator, right at the end that I think would work, but I cannot get this event to invoke my generation code, or if this is even the correct solution:

BiomeDecorator.decorate()
{
...code...

MinecraftForge.EVENT_BUS.post(new DecorateBiomeEvent.Post(currentWorld, randomGenerator, chunk_X, chunk_Z));
}

If you know how to solve this please post and thank you in advance!