adjusting valves on a 440

The first thing I would do, is buy a "pushrod length checker", they are cheap enough on eBay. Your heads and/or block may have been milled in it's previous life. Until you know the "correct" length pushrods, it will never be right. We had the same problem on an RB motor and after buying the length checker and comparing our heads to a virgin set of heads we found out our heads had been cut over .080. We also know that the block had been decked for zero deck. We ordered pushrods from Mancini after we had the correct length, and "viola" no more broken pushrods. I hope this helps you.