I have just started the process of rebuilding my 300K convertable. We pulled the413 non-crossram motor for rebuild. The car will be a summer driver and will bore & hone for new pistons .020 - .060 ? the block goes out next week. I have been told to switch to the 516 or newer heads? I plan to put in hardened seats and some minor porting and have blue print and balance the motor by a local high quality shop. He feels that the 1.60 valve is to small but is the biggest that should be installed with current heads? He thinks a mild purple cam is fine for my application. I know this is a can of worms to open but I would appreciate knowing what others have done, what worked well and what to avoid.