I had this on my 1500 Spit. Mine was re-bored +30. It was worse when hot. It turned out to be the pistons hitting the fire ring on the recessed head gasket.
I took the head off and you could se where it was flattened. I took all four pistons out and had them chamfered.
I remember there was a blog on I think MOSS or CANLEY'S website, or could have been another where they had exactly the same problem after rebuilding a 1500 pit engine.
Danny