340 Max HP Potential with X/J heads?

MOPAROFFICIAL said
Measure your Port volume next time. People don't realize you can just make that port a hell of a lot bigger and even flowing the same will RPM higher. One of the reasons why my production head 4 inch motor will RPM higher than others with production heads is because I got the port volume to 186cc.... that's in the neighborhood of W2's. I'm a nerd about having

To anyone using production iron on a 408-410 CID small block... just open them up. You can do this without touching the short turn if you havent the shaping skill. Raise the roof, knock the link down some, open the pinch, work the straight side wall, shorten the guide 'height and length... lightly blend that all to the bowl.
Part of the reason the rpm head flows more, is mostly due to it being a larger port...the turn is not a gem by any means. Think.

From what I gather, the 915 J's aren't going to flow much different than the eddys once ported so I've gone ahead and bought them for $300. They are untouched from factory from a 71 360. Im in Australia and there aren't any machine shops that are familiar with what I need to do to them now to get them to their 90% potential so I need specifics to tell my machinist exactly what to do to them. Thanks for the help! I'll post a new thread with this question to maybe get some different ideas.