I Agree with the comments on Balsa float but GM use nitrophyl floats in production carburetors for years and I have never heard of it sinking, It is a closed cell material and works great. I use it in all of my old cars. A couple of comments:
1. The mechanical level in the Buick spec only applies to cork floats. All other materials will be different.
2. The only way to get an accurate fuel level setting is using fuel in the bowl at the inlet pressure on the car (if a vacuum tank it is just the head of fuel - about 1/4 psi. I am using an electric pump at 1.5-2 psi. Any pressures over that risk flooding since the needle and seat/float system are not designed for higher pressures.
3. I put small white dot of paint on the inside of the bowl when it is off the cars that is 3/4" below the top of the bowl. When the car is running with the bowl cover off, you can see the actual fuel level. It should not be any higher that that.
4. Nitrophyl floats usually are not flat on the top when set correctly but as long as there is clearance around the float and clearance to the bowl cover, you should be good to go.