Gary,
I looked at the photo, and I think I'm seeing a rather long unsupported tube that the blocks are going to be attached to.
The pull from the sheet has varying upwards, forwards and sideways components, and all these need to be resisted. The best I can think of is a V-shaped bracing tube, from the centre of the horizontal tube, down to the bumkin just aft of the hull. I'm not sure that this is quite enough, it depends on the actual sheet loads and directions, but I think it's enough for your first sailing trials.