Marty,
According to the GM Heritage Site data for
1950 Chevy Trucks the below screenshots provides basic measurement details (see data in pics) for your COE rear springs and bushings.
A further search of parts resources resulted in GM# 369721 as the specific bushing for your application (same bushing for 3600 thru 6400 models).
Originals (see this eBay ad for
a set of 4 NOS bushings) show them to be steel with the slot and GM stamping you mentioned.
A bit of further web searching using the GM# 369721 came back with several vendors offering both steel and brass bushings with the correct dimensions. Too many links to add here, but if you do your own web search you can easily find them. The after market bushings also give the dimension info you needed (matches that given in the GM Heritage data).
Hope this is helpful and you can get the bushings you need.

Dan
EDIT: jimgmc beat me to the draw on posting the info 