The Frankenstein frontier

In southern Ontario there is rust, i owned a 1998 nissan frontier at and got it mechanically sound. Then i bought a parts truck 1999 4x4 se and made one out of two and i now have a 1998.5 Frontier. Its great but i got posted 12 hours away and i realized i need some opinions on some things for my truck namely interested in add on overdrives because they can extend the life of engines while adding fuel economy and spliting all gears making a five speed a ten speed