Either the 700R transmission or get a any chevy manual trans with OD, should bolt up with some modification work this would save fuel more then increase speed.
If I were you I would not worry about speed to much if your from Central America I bet you have the fastest little mule out there compared to all the rest of the traffic.
And then you have to watch for cattle crossing the road all the time so it pays to drive real slow especially at night.
