Joey - You're going to need to invest in a "speedometer adapter" like the one pictured below as probably the easiest way to get your speedometer to read more accurately.
Chevrolet offered a number of these under Parts Group 4.329 - Speedometer Adapter. Those were set with specific correction ratios for adjusting speedometer operation depending on original speedometer drive/driven gears, coupled with the rear axle ratio and tire/rim combinations.
Before you start looking for one, you need to figure out what your adjustment ratio needs to be. Here's a YouTube Video
Finding The Right Ratio that can get you started. Once you figure out the ratio needed, you can start looking for folks selling what you need.
The adapters are available from many vendors, including Jim Carter as
SP249, which is custom built once you tell them what correction you need.*
Hope this is helpful.
*Note - Per the Jim Carter ad, all you need to do is identify your speedometer reading as compared to another vehicle that is confirmed as operating at the correct speed (example - theirs reads 55 and yours 50), and yours will be custom configured to get what you need.
EDIT NOTE - Mike beat me the post button
