I'm looking at doing a repower with a used 2GM20F in my 1970 C&C and am trying to sort all this out as well. I found the article at Pearson Triton #381 Glissando | Building a template for the Yanmar 2GM20F
very helpful. Take a peek. Perhaps we should stay in touch somehow and share what we`ve learned. Let`s use this thread to do so if you`re interested.