You can run the plastic outdrives in 17.5 and 13.5.
With a little modding they work great. I run the HD Aluminum CVD Bones up front with a spool and the plastic outdrives in the rear with the plastic dogbones!
Dremel down the cvd bone pins so they are just under the surface of the outdrive diameter when installed. The next part is the "hard" part. depending on your resources you want an aluminum sleeve made to snugly fit over the outdrive. Anyone with a friend that has lathe access can get these made easily or if you look around you may find a stock size of tubing that fits.
mine are .405" O.D. .375" I.D.