Piston slap is mostly heard when the motor has yet to warm up ,very mellow knock that quiets when warm.
In the bad old days we would pull the liners and turn them 90 degrees. New rings would last about 10 K before breaking all to crap .
I still have a block with the pistons from my old 59.These were taken out in 1970 and re-used in 72 on a real POS. It ran well till wrecked...
Mad dog