John, I rummaged through my bookshelf of factory assembly manuals. I found a reference to the radiator core support I "think" might apply to your cab over truck. This is a 1947-1954 Chevrolet manual but chances are good your GMC is the same or very similar.
This page covers 3800 - 4000 - 5000 - 6000 series Chevy big trucks.
Here is a scan from my manual which will give you an idea of what could be needed, least give you an idea of something like yours.
Big image so you can easily see and read.
Part labeled "Support Cushion" is a rubber pad with two bolt holes. Note more than one is needed. Those pads are the same as for small trucks. I found those pads on the web at several places selling classic truck parts. Small trucks have two supports like in my image, big trucks only have one support. Nonetheless, those rubber pads are the same.
Maybe this will point you in the right direction. Let us know how you do on this.
Billyray
Last edited by 69Cuda; 05/19/2020 9:08 PM. Reason: removed link that contained objectional material, not related to post.
