Is this a chevy or gmc? If chevy and original it will be a 216. If gmc and original it will be a 228. The 1/2 ton you have is already geared with 4.11 gears, and uses a torque tube.

The choices for transmissions you can use is severely limited by that torque tube. You can get the pumpkin out of a 54-62 chevy 1/2 ton and move to an open driveline. 3.90 and 3.38 gears are available for the later open drive axle. Once you make the move to an open driveline, transmission choices increase. The T5 5 speed is very high on the list.

Of course with the need fo speed there is also the need for better braking. The stock brakes are not even close to being up to that task.