I'm going way in here.. and a bit over a guys head.. but... I think the plugs are the most telling. Can't get around that despite all the tranny and timing belt stuff. In order to smash the plugs something had to be in there. The pistons didn't reach them on there own. That would have made quite a bit of noise lol. Also because you had 3 and not one smashed plug we can assume whatever got in there it happened in the intake and was distributed over multiple cylinders. So think stuff rather than "it"? When you dropped rpms vacuum increased. More pull for junk? So next question... if you suddenly smashed 3 plugs what would happen? Might just stop pretty quick. But seize? Not sure about that. Maybe chain reaction into some other part.
Somewhere along the line somebody left some stuff in the intake, carb or heads and it got processed. That made things very unhappy. Thats my guess. Why didn't it happen immediately. Who knows?
I would pull it. Take it all apart and see whats up. At this point what have you got to lose.
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