If the top picture is one of the chainplates, you may well find that you need the cap shrouds running masthead-spreader tip-chainplate; another set of intermediate shrouds running from the spreader bases to the other hole in the chainplates; and the lowers that you already have.
The big rigging suppliers sell a complete standing rigging wire set for many popular boats. You may want to compare the price for the whole set against the price for only the parts you need.
[Edit] Oh yeah. Shrouds go to the sides. Stays are for fore-and-aft support.
It appears you're right. After reading this post I searched the web some more, and found 2 sets of stays coming from the midship chainplate in a few of them. The angles of the stays suggest one could be going spreader bases, while the other would be going through the spreaders to the masthead. Here's are a few pics that are examples (I'm only linking the first because it's wide enough I'm afraid it will distort the page width):
http://www.elgaleonpirata.com.ar/images/avatars/2000198590443484c989ae6.jpg
Something that strikes me as odd about all these pics is that the headstay on all these boats is a plain cable. What I was considering my headstay seemed to be intended to serve as a sort of mast for the jib as well. Unfortunately I am at work right now and none of the pictures I took display this. It was a cable that ran just like I'd expect a forestay to run, but had a sheathing over it that it seemed the base of the jib sale would run through, if that makes any sense. It also seemed to serve as a forestay quite well when my friend and I put the mast up in the yard, it had good tension and support to keep the mast from coming aft, just as the backstay had good tension to keep the mast from coming to the bow. Our only lack of stability seemed to be side to side, which was no doubt because we were connecting the lower shrouds wrong, and were missing the mid and upper shrounds altogether.
So, it's off to start getting an idea what rigging is going to cost me. Any suggested places besides riggingonly to start?
Thanks again! Slowly but surely we (well, I, with and due to your help) seem to be getting there.