Here is a possible source. Axle/Spring Mount Bushing . According to the Master Parts book, they used the same U bolts here for Pass from 38 to 53 and all 1/2 Ton from 47 to 53. I am guessing that maybe the bushings were the same. I am interested in this search too for my 38 Master.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!