I think you may be talking about the cab mounting blocks that fit between the cab & the frame. I think many of the parts suppliers; Jim Carter, Chevs of the 40s, etc have them. They used these on the '40's trucks.

1939 GMC AC-102 Long Bed 1/2-Ton
In the Legacy Stovebolt Gallery
More details in his DITY Bay

1940 Chevy WB Series 1.5-Ton
In the Stovebolt Gallery