The Stovebolt Site Forums The Shop Area Engine Shop 4.10 to 3.54 rear end swap

I currently have 8 lug 4.10 posi in my 66. I now have the chance to purchase a 6 lug 3.54 posi.

Will this swap give me a significant speed increase? And can I change out the gears to increase the speed even more.
I currently have an NP435 trans (reverse on passenger side)