I would pick the 455 smogger or not. Basically the two engines cost about the same to repair and upgrade under most conditions.
The fuel milage with the 3 speed auto between the two, is debatable. Assuming you have a 2:56 rear (even a 3:08), the 455 is not that bad for mpg. You're looking at 12-15 with either engine in a heavy cutlass.
Besides, the 455 will put a grin
on your face everyday.