Photoshop :: Masking 3d Composition

I've built this 3d model and i need some help compositing into the photo

the problem im having is that i need to keep the monkey puzzle tree and the bare tree infront of the building but apart from painting a mask buy hand i i'm struggeling to get any decent results.

Here is a quick mock up i did but i need to mask out the trees accurately

I managed to get the trees where they over lap the sky because the background was contrasting and consistant using the technique below but it removes the bottom of the building and only helped with the trees that intercept the sky line.

I'm using Photshop 6.