my 78 came with a mikuni roundslide carb. it looks like the id of the bore is at 38mm. assuming its not aftermarket it should be the vm38 right? plus where the hell can i find a aircleaner adapter to mount a cv or s&s, tons for hsr carbs but none for vm

 

Guys often measure these incorrectly. Just to be clear, it is measured at the narrowest part of the venturi. Might be easiest to measure from the back end. What i do it cut out a piece of cardboard or stiff paper the size i think the carb is and stick it in there.

 

If it's a round slide Mikuni, it's probably 38 mm. They were a pretty popular aftermarket carb for HDs back, especially in the mid-late '70s. No longer made, and hard to find parts for now. Unless you just gotta have it, it'd probably be better to swap it out for a newer carb.