I just recently replaced the front motor mount on my 1954 3/4 ton truck and yes the "thick washers" were under each of the two outside bolts.

This washer is shown in the Factory Assembly Manual , section 6, Sheet 12.00, 365685 special washer.

In the 1929-1954 Master Parts and Accessories Catalog they are shown under Group .002, Washer, Front Mounting, 2x , 3/8 ID x 1 1/4 OD , part number 365685 . The thickness is not given and I now wish I had measured the ones on my truck before I reassembled the front motor mount. Hope this information helps.

Ron - - Dusty53
1954 Chevy 3604
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