Philippe,
It is great you have a parts book, but there is a site hosted by Keith Hardy that has additional manuals, called Old Online Chevy Manuals. Here is a link from the 1929-1954 manual that may help.
http://chevy.oldcarmanualproject.com/parts/1929_54/095.HTM
You will find that your rubber front mount 3657520 should go with a plate, shield and retainer; all of these are metal. These support the rubber mount, and shield it from engine oil. That rubber mount is not to be used alone. Be sure to look at the next page which has these parts listed as being correct for trucks going back to 1937. It also shows that the rear mount 3702173 can be used on your truck.

The front mount can be hard to find used or NOS. The vendors sell reproductions of these parts that are close enough to the originals to be useful.
Good Luck,
Kent

1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix