For the RS3, the Atlas Trainman shell has hoods that are too tall, to accomodate the vertical China-drive motors. It is most apparent when you look at the cab center windows over the hoods. So a Weaver shell wouldn't fit an Atlas chassis anyway, with the Atlas drivetrain.
I only have an Atlas RS3 because it came with wire grabs instead of molded ones, & the price was too good to refuse. With some work, a Weaver model would yield a more accurate model.