We had a guy locally who was running spacers and spool on his 41 Willys and he sheared his studs off, subsequently rolling the car!
The rear end was a Moser, so the studs were high quality, and the advice he was given is that the loads imposed by turing on the street with the spool on top of the added stress on the studs from the spacers (causing the load to be carried more on the studs than on the axle flange) combined together to cause the failure.
If you want to be 100% bullet proof, the correct wheels or rear end and no spacers is the way to go, but there are plenty of people who sneak by with the spacers, so it comes down to how much risk you are willing to take. Also factored into that is the performance and intended use of the car. At the end of the day it is your @$$ on the line, so do what makes you feel best!