If what you say is, in fact, true, then yeah, you would have to take the block apart. I would be surprised though, because unlike some earlier engines, that one should have an o-ring seal between the block and the bearing. However, anything is possible, especially if the power head has been apart before. I suppose you could always just replace the seal and see where that gets you. If you have to take the block apart after that, at least your seal replacement is already done. I might as well bring this up now, while we are talking about leaks in this area: I have seen more than one 18 crank shaft with pits in the seal area, making it incapable of being able to hold a seal….

OK, well I’m a little confused, but this is a common topic for sure.  It looks like the 67 models did not use the oring, but perhpaps I am not looking at the breakdown accurately.   From your picture, it looks like this engine uses the “recirc style” system with the vacuum port above the seal.  But, I am confused, a very big leak should have been detected there when you did your test.  OMC did make a tool to remove that seal without pulling the powerhead apart, I don’t have the part number for the tool handy though.

So, I will back up a bit, and start with a simple question….  Was the mag plate a mess with excess oil splattered on it and the points?  If not, I would not mess with the seal.  Sounds like you just tried to test the seal only because of what you have read about this issue.  So, if it was mine, and the mag plate was relatively clean, I would not mess with the upper seal…