This might help.
1929 - 1957 Chevrolet Master Parts & Accessories Catalog
Geoup # 4.081 SUPPORT, Transmission Mounting
48-53 1/2 ton part # 3688234
Catalog gives length & width of mount, might be able to find a mount by using part number to find thickness of pad.
Note: 40-47 mounts are almost the same size, there for I believe the pads for all years mentioned are the same size.

Here is one Chevs of the 40's Assembly and Pads only

Pads only Classic Parts of America, Inc. and Assembly

Looking at the assembled mounts from both vendors shows the pads in different configurations.
Stacked or split. Hmm whats stock?

Call them and ask how thick they are and report back.

1954 Chevrolet 3105 Deluxe Panel Truck
w/Factory HYDRA-MATIC