I interpret the book as 365720 is representing only the rubber; the complete assembly is 606427. Your picture showing the box label does not appear to me to be original GM packaging. Perhaps that label is someone's interpretation of the numbers and is perhaps wrong.
Also, between the parts book you have and those on Keith Hardy's site there are missing years. I think it is possible the original mount on your truck, part # 60869 became obsolete and is not represented in the later books, because they didn't want to offer that part # anymore for whatever reason. Some one with more automotive parts knowledge than I might be able to add more coherent thoughts at this point.
I have had good luck and good performance with the aftermarket mounts sold by the vendors on the two trucks that I have removed the engines, my '37 and '50. I replaced front and rear mounts on both trucks.
Kent
