Palomino:
Don't give up. Do you have part number?
Jolly's site shows Hastings number as 2M306 for 351e. Your 351c likely has 4 ring pistons. With that said, that part number looks like it could be special ordered from Hastings with 1-3 months delivery here:.
https://www.amazon.com/Hastings-2M63.../dp/B0049N8MI6
Hope that helps.
Regards Steve
New Orleans