Usually the minimum thickness for the bolt-on style spacer / adapters is 1", sometimes 1-1/4". As mentioned, most times you will end up with the wheels being pushed out past the edge of the fenders on our cars, since you need a high positive offset wheel to start with, and making it stick out another inch or more doesn't help. You won't find much choice in wheels that have enough positive offset to start with, that would still go under an MN-12 once you figure in the spacer thickness.
Bolt-on spacer / adapters are safe, as long as they are correctly torqued to the vehicle hubs, and that torque is rechecked periodically. I have a set on the front of my 68 Cougar, because the 18" wheels I chose weren't available in the correct offset in the width needed for the front of the car. They'll also give me more spoke clearance to the large brakes that are going on. Haven't had any issues with them in 5,000 miles of driving last summer.
Spacers that are hubcentric to the vehicle hub, and the wheel bore, are the best type. Eibach, H&R, and other companies sell the hubcentric spacers for some vehicles. For an MN-12, custom made adapters to 4.5" are available, but are expensive. Unless you really need to get a specific set of wheel on a car, it usually makes more sense to either get some wheels in the correct pattern, or go with the option of a hub swap in our case.
cheers
Ed N.
