Roots blower physical size

How do you determine how big of a snout you need? Is it a one size fits all and you make up alignment issues with spacers or are they very specific to each engine? Can you use a blower that was on a small block chevy and put it on a small block mopar. I know that the intake is different, but the blower is still the same is it not? The more I get into this the more I realize I don't have a clue as to what is really needed.

Jack