Early Sportsters (through mid 70's??????) used a 7/8" neck and so did most of the 45's. I don't really know about the Servi-cars.
I put a Wide Glide front end on a late Sportster for a customer. Even though it came with a 1" neck, I had to ream the neck with a 1" hole saw to get it to clear the stem. I don't know if it was a one time problem or not, it's the only one I ever did.
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.