I can't answer your question. But I was wondering if you have a 235 splash engine, or a 235 full oil engine. Those engines are actually really different, and if you are mixing the head and block from the 2 different engine types that might be your problem.

I believe you could also get the pressed steel rockers and put them in instead of the cast iron rockers. It should be a direct swap.

Sorry for being vague, I'm a gmc guy and never really got into any of the chevy 6s.