Another wheel question Lug nuts this time

[87customcruiser]

Thanks all for the information already given to me about wheels for my Custom Cruiser.
My new question is about lug nuts, I took my wheels to the tire shop to have them mounted up. The shop guys showed me that although the wheels fit the car great, the lug nuts will not hold the center caps on. The shop guys said to try the Chevy dealer that they should have them, but the only ones the parts system showed didn't look like they would work and they were a metric nut. The wagon uses 1/2-20 thread nuts. I'm at a loss as what direction to go, I think there should be a nut that will work but I don't know right off how to find a part number to go by. The nuts don't have to be GM if you know of aftermarket nuts that will work.
I'm attaching a picture of the style wheels and type of caps I'm useing for background. Sorry to keep bothering everyone but I'm very thankful for all the help. TIA

I found after a lot of running around & internet searching a set of lug nuts that are going to work. They are 1/2 20 thread with a 3/4 hex and budge.
I had to bend the anchor tabs on the center caps to make them work, or at least I think they will work. When I am done and have a chance to take a picture or two I will.
If any one wants to try the same thing contact me & I'll try to help you out.

Thanks to all who looked at this post, I am thankfull for all the great advice.