Move the post first. It doesn't matter if it's behind the I-beams. Also, you can move the cross beam with the PVC guides really easy. All it is is a piece of L-beam aluminium that is bolted to the main beams. Unbolt it, drill new holes and bolt back further forward. You may have to cut it down some if it sticks out too far. I agree about the transom overhang, but if it comes down to it, Buy longer bunk boards, or re-bolt them further back on the brackets and that will free up a lot of room to move back. The bunks on my trailer hang over the last cross beam of the trailer frame by about 3 feet. It is pretty much like a cantilever. It looks like you've got a lot of room to move the boards back, up to two feet maybe. I've hauled mine, and a customer boat to Charleston at 65 mph with no problems.

Move the winch back into the "A" frame of the trailer and redrill the holes in your bunks to move them back. I don't like to hang the transom off the bunks due to the large amount of weight going unsupported can eventually put a hook shape in your hull. Ask me how I know!