If you wanted to try using the stepwells on the Atlas cars, use the ones marked 1:48 scale. They are scaled correctly for the car width.
The Comet stepwells are different, however. It might be better to just do specific replacement stepwells for them. If someone were to give me a 'launch order', I could do CAD for them and upload it to Shapeways.
Not sure there is sufficient market to make it worthwhile. The entire underbody of those cars could use some help. I'll check out your 1:48 steps vs the horizon cars and see where the mismatches are.