best machine shop/porter for 340 j heads

For good head work w/excellent flowbench to prove it try Angie at Mopar Engines West, she went to mondello's porting school and know's how to get the most out of your heads. Another excellent choice would be Gromm Racing heads, they have a few mopar guys that have proven their technique as well. Either way you can be assured that it's gonna cost some bread, but that was a given when I fell in love with mopar. You can buy the porting templates from MPP and practice on some old smoggers and then if you like the results, J heads done with the templates are pretty safe bet to be 250cfm +. You can get that out of some Eddies with a gasket match, but that is a much bigger investment than the templates, so for a budget conscious guy you could get pretty fair results with the templates if you aren't afraid of a die grinder and are willing to be patient and careful, you can do it.