I agree with Holly41, There are two many different design factors that go into designing a pump to give you the flow and head you are looking for. Stress on the shaft very with the design such as impeller diameter and rpm. L3/D4 ("L-cubed over D-to-the-fourth") is a measure of pump rotor stiffness, it's ability to resist radial load and to minimize deflection.
It comes from the basic cantilevered beam deflection formula, which you can find in any book on mechanics: y = F x L3 / (3 x E X I), where F is radial load, L is cantilevered length, E modulus of the elasticity of the material, and I is moment of inertia.
The lower L3D4, the less is shaft deflection, which is better for mechanical seal alignment, vibration and shaft whip. If L3D4 becomes too large, a pump shaft can snap, especially if operated close to shut-off, where hydraulic radial loads are excessive and not ballanced on volute type casings. ANSI pumps have L3D4 ratios range from 20 to 120, but newer designs have been introduced with this ratio below 10.0.
Regards Checman