Tim,

I am just going by the Master Parts Book.

http://chevy.oldcarmanualproject.com/1929_57chevyparts/57cmpc0146.htm

The Master Parts Book shows 3/8" pulley for 53-55 Corvettes and the same part number for all 6-cylinder 54 passenger and commercial. The 5/8" pulley is however shown for the the 53-55 Utility Body 235/261.

Tim