Nothing wrong with the original type master cylinder if it is in working order. The reason that the dual circuit is considered safer is that it splits the system into separate front and rear circuits. With the system split, if a leak in the system occures, like say a blown line or wheel cylinder, you still have the other circuit to provide braking power.
Converting over will depend on what you plan to do with the truck. If you are going to do a disk conversion, a dual system is pretty much manditory. You would also want to consider it if you plan to use the truck for highway/long distance driving. for short distance, car show, occasional weekend cruise type use, the original MC will be just fine.
I will admit, I am still using the original style on mine, and have had no problems with t, either in town or on the highway.
