I was able to find some add'l info on tire sizes. Looks like a 255/45 18 (27.1") and a 285/40 18 (27.0") should work with my setup. The truck at this link -
https://goo.gl/qXH5gH
... is running 285/40 18s at all 4 corners. Looks great to me, although I don't know how mine will look sitting up a few inches higher.