Them big m's are actually very good heads. I had them on my 548 everyone said they were way to big, so I swapped to dart 355's cnc'd and lost a few hundreths. They would be excellent on a 598

Kinda funny you said that hammertime. When I was building my 540 I found a set of Big M heads and I had two different builders tell me that they were too big and that they would be good for either a 572+ cu in motor or a smaller supercharged motor. I bought the Dart 355 cnc heads too, ran them two seasons, and then sold them. I was not happy with 355s at all.