Are you sure it is ft3? Because when you say ft3, it refers to the thyroid, maybe you're referring to fluidounce, right? Or fluidram.
let's just change the ft3 to fluidounce,
First, we must know the conversion
1 mL=0.03 fl.oz
Therefore, the equivalent of 18.7g/mL to lbs/fl.oz is 1.4 lbs/fl.oz.