You have two options:
1. Cut (shave) the studs shorter. It is how guys are using thin bolt on spacers and wheels without pockets. I cut 10 in an hour with a hacksaw; my arm was tired. This is the cheapest option.
2. Install the 20mm spacers on front and get 25mm spacers for the rear. You could also go 20mm all around. This is the simplest option.
Option B: Have a beer! Sell the spacers!
