My question is will they fit in the fender skirts on the roadmaster sedan?
is the sidewall enough to support the weight of our cars
yes, though not really with 6 people and a trunk full of ammo.
with enough coverage to kind of cover the lip of the wheels?
No. You will get zero protection for the wheels from 275/40's, but they do lower the car nicely and improve the handling and braking. Ride suffers noticably but not obnoxious. With the right suspension the car will handle great.