If you think the pushrods from a late '54 Merc 256 would fit, I have nearly a full set of those old skinny ones.
My highly accurate tape measure says they're mostly 8 & 3/16 overall length. A few seem to be a hair shorter.
Cool beans! How muchy? I think there's a good chance they'll fit, my 292 is a pretty early one. I doubt all of them were EXACTLY the same length from the git go due to manufacturing tolerances.
I'd prefer to send you 2 or 3 so you can pick the best one, I haven't looked closely at them in about 8 years.
Send me a private message and we'll work it out.