Valve / Piston Clearance in 383

I don't think it's anywhere near that far down. What you're seeing is the taper on the edge of the piston, and the taper on the edge of the bore. It looks more to me to be .020-.030" down if that. I'd imagine that engine had to have some work done to get it like that but it's a good thing as long as the stock type pistons don't hit anything.
I agree in keeping the lift and duration conservative - and modern profiles tend to run less piston to valve clearance than the older stuff.
I'll also add - you may want to get some cylinder pressure readings. If the cam is in retarded (grind poorly) or straight up it will be soft on the bottom and you will see lower cylinder pressures from it.