probably too late in the game for this round, but a curved track seems most sensible to me. This allows the weight to always be on the arm axle/wheels instead of having to transfer onto it shockingly once the height is right. Then, with the curved backswing, less jouncing/rebounding, or at least, what DOES happen, will be less shock load on the whole shebang because it’ll be smooth.
http://www.supertrebs.com/Merlin.html
But as always, I may just be looking at it wrong…