Far as I'm concerned, you might as well find a friendly Treasure Yard, and buy used ones. I tried the Poly inserts, too. They're not made right for K-series trucks. The steel inserts in the Poly are angled instead of vertical; and the Poly insert itself is too huge.
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I have NO IDEA why building a decent engine mount has to be so impossible. I put my 300,000-mile, 30-year-old mounts back in the truck because I couldn't find an acceptable "new" solution. If there's a decent mount or Poly insert to be had...I don't know about it.