I would keep them as level as I could..........otherwise it could mess with the float levels in the carbs.........if the carbs ain't level that means the engine isn't level either............I can see where you are going with your build (hardtail and 6" over front end)............but..........how do you know you need a 6" over front end???.....I would do the hardtail first........keeping the lower rails parallel to the ground, and in turn keeping the engine level.......after that, measure for the length of the front end..........just my opinion though......................