From an AMC forum:
Re: 304 Overbore limits
Reply #6 - 11/30/06 at 02:49:43
According to "Performance American Style", the max limit you can bore a 304 is .125 over (3.75 to 3.875). If you do this, you can then use stock bore 4.0L pistons. I did this to my 304 and bought coated hyper pistons, but haven't assembled it yet for testing.