I just rebuilt my 300 six, but can't remember what kind of key is used in the crank. If it's a Woodruff key, maybe one of these will work.
http://store.summitracing.com/egnsea...alse&N=700+115

If it's a straight key, call or email these guys and explain what you need.
http://www.mcmaster.com/#contact/=ilomx