1) There are multiple threads on how to convert to 12 volts. I do not believe anyone sells a complete kit, but it is simple a matter of assembling a few components.
2) You might be able to put in seat belts with shoulder harness, but even the factories have a hard time getting the airbags right. They can be deadly if done wrong.
3) Unless you are going with low-rider hydraulics, you will not need more than 60 amps.
4) You imply that you already have overdrive. If that is the case, you might ask another question on just how to convert it from 6 to 12 volts.
I suggest that you research the threads on your topics, and then come back with specific questions on details that you do not understand. That will get you more useful responses than the shotgun approach.
