Yes, you are using the same size thread size as the stock sled. And that's the problem. The stock thread size is a smaller size than many standard mounts of that type. I have several HU's (Kenwood, Soundstream, MINI stock) that all use a larger fastener there. To be honest, the Empeg sled is the only one that I recall seeing with that size fastener.

Did the Empeg sled come with a double ended bolt and rubber plug that goes there? I don't recall seeing one.

While I'm on the subject of the new sled.... Excellent job on fabrication. The copy of the original is just about flawless. I even noticed that the cutout for the connector plug in the rear has the same "flaw" as the orginal. The original cutout was a smidge to small to let the connecter easily seat in the cutout. Wnen the screws are tightened, it forces it into place. Your copy is the same way. Impressive attention to detail.

All in all, a great sled.