Factory '68/'69 - 383 Piston: 1.932" CH, no valve relief.

Reasonable choices:

Speed Pro 2315: 1.920" CH, flat top, no valve relief

Icon IC687: 1.912" CH, small dome and valve relief, positive 4.5 cc, 674 grams

Diamond 51900: 1.916" CH, flat top w/ relief, negative 3.0 cc., 630 grams