Hi Vance. From my limited experience with the K boats, I would much prefer the saddle tank option. It is lots more stable, and easier to fabricate and attach. It may cut down on top speed a bit if the tanks aren't sleek enough, but by using FRP that deficiency can be easily eliminated.
An additional benefit ( requiring a little more work) is to have the saddle tanks split (front and back) so surface angle can be adjusted. You could then carry a little water in the rear tanks when towing to keep the nose up. Also, if "riding the bubble" near the bottom you could use them to adjust dive angle slightly without complicated dive planes and associated linkages.
I made my fairings fit the hull exactly by applying wax to the steel hull, attaching the mold and glassing the whole thing while attached to the hull. When I popped it off and sanded them, they fit the hull within an 1/8 inch on all the curves. Very easy.
In your case, I would wax the hull, glass it for the back side of the tanks, then attach the molds and glass again. After the first layer was complete, remove the FRP tank, cut it in half and remove the mold from the inside, put the two halves back together and add the remaining layers of glass. Build it up to however thick you want ( 5 to 7 layers) and finish with gel coat. You could get a nice smooth shape for the tanks with a leading edge for surface travel, a flat "deck" surface with grip surface, and make it look as sleek as you wanted.
If you think you'd like to try this, I'd be happy to write you a more detailed description, maybe sketch up some ideas, and help any way I can. Frank D.