The general rule of thumb for thread engagement is 1 1/2 times the diameter of the threads...
Example:
a 1/2" thread times 1.5 is 3/4". So you should try to get around 3/4" of thread depth between the lugs and the nuts, and still have the bolt seating on the face of the rim...
How long are the studs?
How thick is the rim around where the lug nuts go?
Do the nuts clamp at the bottom of the nuts, or around the base of the nut where the washers go???