What is your budget or access to auto parts? What will you use it for? Does it need to float? Do you have a year to work on it? Typically you will need the open front diff off a independant suspension 4x4, Japanese mini trucks will have lower gear ratios. You need to move disc brakes inboard for braked steering then short CV shafts to your drive wheels. ATV Tires allow you to pump up the tires to hold the tracks or else you will need a track tensioning system added. Sounds like a neat project.