Welcome aboard Epops. I would be concerned with a bouncy floor if things are under sized. One guy chopping or pounding away and the other trying to hold the square in an lousy position, the bouncy floor will only add to the already lousy position. As a student of such matters we know the loads we choose for the direct use of the structure can and does change in a few years. A heavier system will ensure that change in the future will be met. I would not have any issues pushing those 4x6 out to 4x7 or even the 5x7. Also consider the center dove tail to be exchange for a tusk or soffit tenon and tenon it into the face of the center beam, this will keep the top cord of that beam in tack. What size tractor will you park in there? Got some spare engines that also need cover lets put them in the shed.