I got a chance yesterday to do the pushrod experiment. I pulled the pushrods from #6 and the noise continued. so I guess its probably piston slap, as beltfed experienced. I think I'm going to consider it a minor issue and proceed with installation. I've got a line on another low-mileage 250 that I can build up in parallel with debugging the install in the 53. Then I can swap in the new motor after I've gotten all the other issues solved.
Thanks for all the input, guys.
Leon
