Futons are build a lot of different ways; your question is going to be difficult to answer at a distance, and impossible without pictures.

My guess is that, with needing to make custom cushions and significantly modify the frame, you may be better off to sell the thing and buy a new couch.

I just took a closer look at the futon frame. DH has already screwed 3 lengthwise slats over the ones that already run the depth of it. He did it so that the mattress would lay level, like a bed, rather than make him lay at a side to side tilt, ever rolling toward the back.

So, the frame already sits level. I thought I still had the rolled up towels and blankets stuffed in there to level it up.

Now I only have to deal with the back being too angled. Maybe just hand out throw pillow and let folks use those to adjust their own "tilt".