Duane,

I'ts been a long while since I worked on my brothers 58 Merc, so I could be wrong. Still, I do believe that the 312 was available in 58 Mercs that were sold in Canada, where I grew up, but I'd have to look up some old info that my brother has on them.
I do know that the 430 was only engine available in the Parklane that year, with no other option. The other Mercs could be had with the 383., but not the 430. Agreed?

John