"IF" (and a big "if" needs to be emphasized) the client is demanding that long a vessel is supported at multiple point by saddle supports, then the client must be willing to pay for the considerable civil engineering and structural design needed to ensure all 6 of the saddles are equally uniformly supported underneath and that all six saddle support will move exactly in the required straight line and parallel to the tank centerline over the life of the horizontal pressure vessel.
It is not enough to "assume" the tank centerline is supported by six "equal and constant" forces at each saddle, but the "bottom" of each saddle must be supported somehow on the dirt and rock and gravel of the site to keep all six saddle foundations from moving with respect to each other. Else, the middle of 6 supports moves up, or one end moves down, or both emd move down but the middle remains fixed, and the tank is bent is half as it tries to cantilever the end, lifting not only the tank steel, but every "low" saddle and that saddle's foundation.
Stick with two supports. ribs on the tank (external) can help, but are not often used. Usually, it's cheaper to increase tank walls slightly.
