Are girdles generally added for big rpm or HP or torque

Jadaharabi said
Doesn't the Mopar small block windage tray kind of act like a girdle on the Mains that It Bolts up to?

No. To flimsy to do anything except what it was made for. The main caps, 3 & 5, have main caps bolts provisions to accept the tiny attaching screws for the windage tray.