Valve / Piston Clearance in 383

Just an update - Took the Duster out racing a couple of weeks ago on an extremely hot day around 35 degrees (around 95-100 degrees for those in Nth America) and it ran a 13.79 at 103 mph (2.109 60ft). I was a little disappointed as thought it would run a little quicker than that but the weather didn't help at all. I reckon it's got a low 13 in it with good weather. My shifter cable was causing me grief too and it ended up snapping later on. First time I've ever raced the car as well. Really want to get the car into the 12's. Any advice on what I should do here? Cam and heads?