Sounds like a great project... and a stern-steerer, nonetheless! As for the runners:
"...that extend beyond the wooden bottom of the runners by about 1/4"."
Not your problem this time, but a good idea would be to have more than 1/4" of bare runner stock extending down. On clean ice, your arrangement would not be a problem, but any crusty stuff, or patches of snow, and your 2x4 stock would hit that, and interfere.
"I flat filed the steel blades that are mounted on the runners. Do they need to be hollow ground like skate blades?"
No, not at all like skate blades. They should be sharpened to between about 45 and 55 degrees. The racers may have different specs... but 45 degrees is a good all-around angle. 55 is good if your ice is always harder. Cuts in better.
From front to back, the runner will work fine dead straight. But some feel a slight bit of rocker is a good idea... I don't do this on the Icester (
http://www.icester.net)
"I suspect I need to move my center of gravity aft to
put more weight on the rudder."
I suppose this could help. But if your runners are sharp, it takes quite a force to break them loose, sideways, from the ice. The biggest factor of all could be the very fact that you have chosen a stern-steering design. What was called "spinning", or the breaking loose of the steering runner, was a common problem on stern steering iceboats. But this was not as much a weight distibution problem as one resulting from the relationship of lateral force (the sideways push the sail exerts) and the center of lateral resistance (the runners). On a front steerer, the lateral force is closer to the double runners on the crossplank. On a stern-steerer, since center of the sail is usually behind the crossplank, the lateral force acts unduly on the steering runner... putting a huge demand on it. It must steer, and stop a major sideways force.
Later stern-steerers, you will notice, moved the mast forward of the crossplank, in an attempt to have the crossplank runners take more of this sideways force. But front steering iceboats finally solved the problem of spinning.
"Can someone recommend some suggestions or reading for me to gain more insight on this?"
Find a copy of the boat, "Iceboating", by S.Calhoun Smith. It was published in the 60's, and is long out of print. It has many basics, and not much of the standard practices have changed... outside of new materials like carbon fiber and such. And again, racers have a whole different way of looking at the sport.
"I recall seeing a very small tubular iceboat years ago but see nothing mentioned about them at the websites I visited."
My guess would be that was a Lockley Skimmer. The Lockley design was bought by a guy in Pennsylvania, who was still selling the boat under the name "Skimmer 45" (for the 45 square foot sail), for about $1,000. I last saw ads for the boat in the mid-90's in Messing About in Boats, but I do not know if he is still in business.
"I'm also curious as to the possibility of switching
over to a sailboard style sail but..."
Can't recommend a sailboard rig... I've done that, and it works after a fasion. But the problem is, "how to attach the stays". They have to be attached to the top of the mast... I did this by fabricating a "cap" which sat like a hat up there... it had the three guys attached. Of course this is not at the major area of stress... which is about 2/3 to 3/4 up the mast. So I snapped a nice sailboard mast once in a brisk wind. It was loud and scary!
"...but the Sunflower sail is basically a large letter A shape turned sideways and
the pull of the sail is off centered to the iceboat and would repeatedly pull my whole boat sideways..."
Yes, this is a factor, too. An iceboat demands a much flatter sail shape than a softwater boat, and the Sunflower lateen rig is hard to flatten. So as you surmise, it will cause a great deal of lateral force for any nice forward force desired. If you can figure out a way to draw the sail tighter and flatter, that will help alot.
Another important factor is that the two runners on the ends of your crossplank MUST BE aligned perfectly to each other. Bring an accurate tape measure to the boat with you next time, and adjust the two runners to each other as fine as you can get them. The smallest "angle out" or "angle in" of these runners, and they fight each other each inch of ice you travel.
Let us know how it goes. I love projects like this. They are great fun. If you have shots of the boat, I'll post a page on the "one off" design section at the Icester site. Rich.