best machine shop/porter for 340 j heads

Porter racing heads built my 340 which would run the ETs you are looking for with the proper chassis set up. It was never intended to be a street motor though.

I do agree with all of the above though. any of the shop mention can do what you need.

A stroker will make it more streetable but a 420+ cube small block will probably need more air flow then a set of J heads can support no matter who ports them.