Although the CMP parts books list the transmission casing as being Pn 590524, it seems that 590474 is a common alternative.
I have 2 Chev CMP gearboxes with the casing number 590474.
The mainshaft 3rd gear is
pn 590493, while the countershaft 3rd gear is
pn 590502, both available NOS from a variety of sources.