Speedo correction is a software issue. Fairly easy to do with 4:10 gears, apparently more difficult with 4:56 gears.
There are many threads discussing gear swaps. Factory 3:73 on a 6 speed is from American Axle. I managed to locate a set of AA 4:10 gears, had the GM dealership do the swap. Deadly quiet, great improvement to the truck.
AA 4:56 gears were available initially, but the supply ran out sometime in 2005, because of the number of SSRs making the swap. I believe the gears originally came from mid-70s pickups, and had not been used since then.
After discussing the project with some who had already done it, the concensus was that 4:56 gears were a great improvement, but overkill if a supercharger was to be part of the swap.
Now that I've had the gears for 11 years, and added a supercharger two years ago, I'm happy enough with the swap that I won't spend additional money doing another gear change. When I first did the swap, the first few runs were downright exciting. Now that I'm used to the improvement, I would like more, but can't justify the additional cost.
If I did, I think I would be looking for a set of 4:30 gears - a bit more performance without making 1st quite as redundant as it apparently is with 4:56 gears on a supercharger.