1968 383 piston replacement

Like Demon said... no EXACT replacement available.
However,
You could try the KB162 as about the closest thing to a stock replacement Piston as you will find ?
It is pretty close to the same height as the stock Piston, but the KB162 has Valve Reliefs(add 6CC's) which lowers the Compression Ratio to 9.2:1, and if you are using 906 Iron Heads(88CC's) not a bad idea for Todays Pump Gas on a stock Cam ?
Then,
if you decide to start adding a bigger Camshaft...you can always get your Machine Shop to deck the Block(.020" room to zero Deck) and bring it up to 9.5:1. which should still be fine on 91 Octane with Camshafts in the mid 220's @ .050 Duration.