On the 68-68 383 4bbl motors the blueprint specs were a +.020 on the piston deck height. If you run the steel shim headgasket you have a real good chance of having piston to head contact.
Many years ago I picked up a 68 ChargerR/T for parts. It had the same situation, when I tore the motor down I found some clown had installed 516's. The pistons had been kissing the heads. Dave