I have bought a 435 hp crate engine

Keep the turbo 400 tranny and have it rebuilt but, $3000 seems a little excessive. It should not need to be re-engineered, and will easily handle your 435 HP crate engine. With regard to your gearing, I would not go outside of the 3.83 - 4.11 range. Running the 4.11s you'll need a lot of tire to keep the tires from spinning. In reference to the "1500" in your title line, if this is a truck, your going to have to put a good bit of weight in the back end to keep the tires from spinning.