If you do the swap, you will pick up about 20% top end speed if you do the math. You can add to this by increasing the diameter of your tires. When I changed out the 4.56 to a 4.10 on my '57 3600, I gained 10% and picked up another 10% by switching from 17.5" rims with bias ply tires to 19.5" rims with 8R19.5 radials. The truck runs very comfortably with this combination.
The differential swap is an easy Saturday morning project. Unless you have arms of steel, I recommend having a tranny jack with some wood scraps to use as shims to make the swap.
I had some trouble finding all the correct gaskets when I did the swap about three years ago. If you do a search on this site, I posted all the correct current gasket reference numbers to save someone else the trouble.
Good luck,
Matt
