You can get a -8 an female to -6 an male reducer and then screw it onto a bulkhead fitting. But I doubt you can get it as one fitting. And here is a second way that is more compact. In stead of a Bulkhead fitting you can screw the reducer onto a -8 Male X -8 Male fitting. But it has to be tight to make it seal. So you would have to adjust the clearance or tightness on the sheetmetal that is passes through by adding washers or sanding the bottom of the reducer fitting as I already have on this one. It would not hurt if the whole thing was slightly loose on the sheetmetal as long as you backed it with a wrench when you tightened other fittings/lines onto it. Which you should anyways.

Ron.