This is a tough subject to get right because there are a lot of options on how to go about it.
I would have the rear axle gears checked for wear, or do it myself, before deciding to keep the original axle. I personally would keep the axle cause it fits the fender wells the best compared to an axle swap. Changing out the gear set can be expensive and requires an experienced mechanic to get the mesh correct.
These old trucks were designed for a time when 55 mph was about as fast as you could go on the roads back then. To get more speed you need to make sure your brakes are in good shape or update the front to disc (a matter of taste). The steering needs to be tight with minimal wandering. I mention this because white knuckle driving is not fun.
