Careful using that builder guide, it has a lot of mistakes, such as swapping the brake rotor specs for the F350 DRW and F450, listing the F450 rear as an S111 instead of the S130....AND saying the spring packs are the same.
I'd be surprised, assuming they would bolt up, if the F450 springs would offer more payload capacity. They have fewer leafs (although I believe they are thicker) than the F350 DRW, but are rated for 9100 vs 9650 on the F350.