I have had 1.25" thick hub centric wheel adaptors (for spacing) on my 96 chevy 1500 4X4 for over 60,000 miles with no problems. And that is with oversized tires too.
I also just installed .60" hub centric wheel spacers on my 90 vette (rear only). It required longer studs but because the spacers are hub centric, stud shear from road impacts is very unlikely. I suppose shear from wheel torque is possible, but I don't think my stock L98 and rubber is capable generating that much torque.

 

I have been trying to find a pair of hub centric adapters in the .5" to .6" range, but I haven't had any luck finding them. May I ask where you got them and how much? Thanks.

 

I have 1" adaptors at all four corners and they seem to be holding up very well. The vette does not vibrate anymore than it did without the adaptors.
The fastest that I have been is 180mph and the front spoiler broke.
Make sure that you get a good set of "Adaptors" and NO SPACERS.

 

You won't find .5 to .6" "wheel adaptors". At that thickness you will only find "wheel spacers". The difference between spacers and adaptors is that adaptors bolt onto your existing lug studs and then have an addition set of studs to bolt the wheel to. Spacers are just a plate that has clearance holes for you existing lug studs and it is just sandwiched between the wheel and the hub.
I generally prefer adaptors but I was only looking to space my wheels out just a little for a better look. I got my spacers at http://www.wheeladapter.com/home.asp. They are .60" thick. They are very good quality and the thing about these is they are hub centric. Maybe you already know, but anything you put on you car you want to be hub centric.
The spacers I bought cost $150 for the pair including shipping. But you have more work to do besides just buying them and slapping them on. Because they move your wheel out .60" means you original studs are going to be too short. What I did was to purchase new longer studs. GM has an aftermarket PN for the longer studs its 22551491. I ordered them online at gmpartsdirect.com. They were $1.40 each plus shipping.
But wait, you can't just buy them and put them in either. They are about 3" long which is too long. At least for my application they were too long. I have the original wheels on my '90 with the plastic lug nut caps. You will probably have to cut them down to the length you need.
It was a little bit of work but I am very satified with the look and performance. No vibration at all.