This has been discussed many times before. It can be done (and has), but it requires custom machining of a lot of the14bff parts and usually ends up costing more than the front 8-lug conversion and new 8-lug wheels.
There is no bolt-on wheel adapter for this. I looked for somebody to make them for me and couldn't find a company that would. It would have to be very thick and it's strength would be questionable since some of the bolt holes have to be very close to each other. Plus, you 6-lug rims would have to have the center hole enlarged out pretty much into the lug nut area, which is obviously not good.
Another option is to use a 14b s/f in the rear since they can be converted to 6-lug easier. Of course they aren't as strong.