I believe they are 3/16" thick, the key is to get the setting close to the measurement shown on the right side of the FAM page. I have parted/owned 32 of these trucks on a few I have found shims which I believe were from the factory on the mounts. It helps to get it close so all the things that come into or out of the cab can be aligned. Also this system is designed as a 5-point support for mid-49 thru 54 (47 to mid-49 was 4) sheet metal purposes (4 on the cab and 1 at the radiator).
https://www.flickr.com/photos/advance-design_parts_co/14813055277/in/set-72157631872034068
https://www.flickr.com/photos/advance-design_parts_co/8350452080/in/set-72157632440242503

KMAC