I'm not sure what you mean by "true flat". If you mean just a square flat bottom to the runners... not at all. But no, not an inverted "V", either. Runners are a "V" on the bottom. The pressure from the weight on the ice, even at high speeds, actually melts the ice as it passes over... just where it contacts. Very small amount, of course. The runner is riding in the resulting (teeny tiny) channel thus formed as you move forward. It does seem incredible... but think about it for a moment, and you will realize that if it did not do this, how would you get any lateral resistance? A boat with runners NOT forming such a groove would just slid sideways, and all over. Runners have a "V". I have always simply used a 45 degree cut, but I understand that some use more like 55 degrees... maybe in racing boats?
As for going over 100 mph... that would be a real difficult achievement while you are trying to put together a boat on the fly, while learning from the ground up, as you are. And I will repeat, it could be very, very dangerous. With the Icester, I had several years and several boats before I had a design which I could release as workable. I had boats break up under me at less than 50 mph... and stuff gets really hairy, really quickly. For one thing, which most new to iceboating are not aware of... there are tremendous downward forces on the boat. The mast is pushing down with sometimes hundreds of pounds when you are moving forward. The boat is a bridge between the front and rear runners. Ever notice the wire below a DN fuselage, running from front to rear? It is supported on a small strut under the mast. This was added to the design because these downward forces would crush an un-cabled fuselage.
So is your fuselage, from front to rear, a strong enough bridge? That is just one example... an iceboat is an overstressed, lightweight, abused bit of sticks and wires... pounding across the ice while frozen solid.
More power to you if you can pull it off... If you can learn about what is normally done, modify it all for your needs, then apply it to a boat you build... and then manage to top 100 mph. But personally, I would recommend building a smaller boat first, with less forces, less speed.
But you are enthusastic, and that's great. Perhaps you could find S.Smith Calhoun's 1960 book "Iceboating". You could also purchase plans for a DN or a Nite, which are reasonably priced, and see the way iceboats are designed (see... not plugging my own boat there!). There are other publications... WoodenBoat magazine... issues #38 and #39, I think, have the history of iceboating, which has much information. They still offer them as back issues... or try eBay.
I don't want to sound discouraging, because I really love projects like yours. My first boat was "only" a 60 mph "V" hull design, but it was a gas. BUT... even getting that crude first attempt up and running, without killing myself, I did alot of research on how it all worked. Be really, really, careful.