It has been my experience that any time you put a smaller carb on a bigger engine you run into an underfueling problem
Check the bore of the carb to see if it is the same diameter as the manifold.If they are the same,you shouldn't have much problem.It would likely be a matter of adjustment.
As to the stud size,I am not sure.What you could do is make some kind of sleeve to take up the extra space and prevent any movement,and help assure that the carb is properly set in place.It can be hard to get them properly centered with to much play on the studs.
Best option is to get the proper carb.