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Paul,

Right, yours is the precisely the situation Joe is discussing a solution for. Your early 1970s lower bolts are in the position of the "blind" holes of later models, where the hole was a threaded blind hole into the casting of the motor mounting bracket.

You could raise your motor one or two bolt holes by using the strategies we are discussing here. You may well ventilate if your motor is raised two bolt holes *AND* you use the same prop you have now, but a good stainless prop will work fine two holes up.

The holes in the transom on a boat like your would look like these two drawings depending on if you went up one hole or two holes: