best machine shop/porter for 340 j heads

IMHO, why spend the money porting a set of iron heads.

Get a set of Ede's or other aluminum head and go from there.

Being in PA, Ryan at Shady Dell is very good to work with. Brian at Indio Motor Machine, Indio, CA is also very good but on the other side of the country.

Shipping a set of Iron heads cross country is EXPENSIVE. Looking at about $75 each way, or 3/4 the cost of a set of new valves...

If you go with a stroker, it needs to breathe.