[tubman]

If by "slow", you mean that the top speed is limited and it is unsuitable for highway use, the problem is the rear-end ratio not the transmission. All contemporary Ford transmissions that would fit your vehicle have a top gear ratio of 1:1, meaning they would all have the same top speed. You need to change out the rear end for one with a higher (lower numerically lower) rear end. For example, change from a 4:11 to a 3:54. You can change the gear set in the original rear end or get a complete assembly with a more suitable ration out of another similar vehicle. Or you could do some modification to the vehicle to replace the existing rear end with one out of a later model vehicle with a more suitable ratio.

You also could change the transmission for one from a later vehicle with an overdrive top gear, such as a Borg-Warner T5. Many have done this, but it a relatively complicated swap requiring aftermarket parts and some custom fabrication. Since your pickup has an open rear drive instead of a torque-tube (like the cars) changing the rear end is a little less difficult.

It takes a little research and planning to come up with a satisfactory combination. Once you decide which way you want to go, there is a lot of information available (here and other places) that will be helpful in your obtaining the desired outcome.