.60 overbore on a 1968 340

My engine builder is recommending a .60 overbore on my 1968 340, it's at .40 now. I know the books say you can do this but does anyone have any first hand experience with this? Pros and cons? The car is a 1968 Barracuda, building a nice hotter than stock weekend and show driver, original X heads, 3.55 gears, automatic, LD340 intake, Holley 600 carb.