Using the engine mounts bolts is fine. The rivets old the the halves together during assembly at the factory when the truck is built, allows for faster assembly. It is very common to remove these rivets when changing engine mounts. You can use zip ties to hold it somewhat together while you put the mounts back in, once the bolts are started or half way bolted down, cut the zip ties off, tighten the bolts and you will be down the road motors. I did mine over 2 years ago, haven't had a single issue yet, that includes 2 different engines being swapped in and out.
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