When enough of the wheel studs remains to safely engage the lug nuts and provide enough and proper force to retain the wheel safely. Is there enough after taking away 3/8"?
Bolt on wheel adapters are a much better choice than spacers IMHO, but I think 3/4" is the minimum thickness for an adapter.
If the lug spacing isn't the same (not close, the same), I wouldn't use the wheels, period.
