Ok, here is my input. A 4l60e will not last behind a 6.0 for very long even in stock form. The torque will destroy it. I 6.0/4l80e swapped my silverado and its easy. The 80e can be had for around 600 or maybe less but you have to go out and find them. I paid 700 or 800 for mine with 77k on it (it was MINT) and had a transgo HD-2 kick install and re-freshed the clutches. All in all I have $1k in it (for now.... something is going in it or the Torque Converter... bad tune).
They come behind the 6.0 and 4.8 in the 3500 cargo vans. All i did to hook it up was run 2 wires for the 2nd VSS sensor to the computer and re-pin the main hook-up and segment swap it via HPtuners. It's that simple. You mentioned 450rwhp was your goal, it will handle that no problem in stock form
The 80e has my vote. As far as gearing, its the same as a th400 just with OD 2.482, 1.482, 1, .75. If you stick with a 26"-28" tire 3.73's or 3.90's would be fine to get you movin pretty good.
Hope that helped