A fair question is, "why buy several boxes of parts then take time to assemble trans from a bare case?"
Answer, simply my style and desire. Often you will hear the comment, "don't build a race car, buy a ready to race package, save money"
For some racers that is reasonable advice. For me, much of my pleasure is in the building. In addition, my brain is wired for problem solving and creativity. So if you are among the build it yourself class, good for you! It is mighty fine therapy and O the lessons learned
