I think I've decided what I'm doing:
4x6 PT retaining wall between piers (laying on 6" side) with gravel base and rebar every 4 ft. None of the PT will be below grade.
2x6 sill plate all 4 sides.
4x6 rim joist all 4 sides.
Fill with gravel and tamp.
(2)4x6 beams with 2x8 footings (similar to what FMC1959 describes in
post #161) dividing floor into three 8' x 12' sections for easy plywood install.
2x6 8' floor joists 16" OC parallel to walls.
3/4" PT plywood flooring.
RickP - Your Joist Span Calculator link is useful for someone with the experience to use it (not me). Is 92" an acceptable span for 2x6 joists at 16" OC?
I figured it out. 2x6 is no problem at 8ft.