Not only is the stem larger in diameter, it is longer! A Servi-Car that uses A wide-glide has an extended neck casting to enable it to work with a standard Big Twin stem. The overall length of a Sportster neck is like a 45, however the 45 has an extended top nut that Sportsters do not, and no such nut exists for Big Twins. The casting holes in the neck are also smaller so Big Twin Cups won't fit a Sporty neck. about the only way to make it work is to fabricate a stem for the BT trees that is made to sportster dimensions.
Robbie