Hey there larry I do have some thoughts on what works well on the Seniors. I have set up four or five seniors of my own and even more of them of friends and students.
With a typical Sig Kits usually come in at under seven pounds and the Sig arfs just a tad more. The CMP Seniors (called Super Seniors) that were sold during that long period that Sig had none during their change in ownership actually were coming in as high as 11.5 pounds. All the above I am talking about without any mods to speak of and typical engines and on wheels.
Now Kit bashed and or heavy mods as you know can sure build weight. My Quad Cadet when on floats is just under fourteen pounds The Catalina Cadet with a full load of 128 ounces of gasoline hits 17 pounds and the Turbine Cadet on floats with 8 ounces of glow fuel and 64 ounces of kerosene Is right at 15 pounds.
Larry your Kubdet at 12 pounds will do quite well with a typical 48 inch floats in fact its likely thats even more than necessary. I have always preferred that if I error in choosing float size/displacement that's it much better to error to large.
The smallest I have ever used on a light Senior is 32 inch butt this can be tricky and its very easy to bury one of the bows when making a taxi turn back down wind.
The pictures are some of the floats I use, these being on varous Seniors.
The Yellow floats on the far left are floats by the builder of the Bushwhacker and my second pair, he produced plans and there quite a few sets have been built around this region. They are 45 inchs long and on the Bushwhacker that combo I think is at least 23 pounds. The black pair are 42 inchs long and ones I whacked out of foam and sheeted The have considerably less buoyancy than the 45 inchers but do the job adequately on the Quad Cadet. The next silver pair are 36 inchers (in this case Goldberg kit) and my go to float for the light weight Seniors. The last silver pair on the right are the 32 inchers, ones a little short for the Seniors.
John