My 340 in my 66 developed issues :-(

For the money your going to spend on the headers, look into 340 hipo logs. Sure they are not headers, but they almost fit, maybe a little grinding but I never ran them with a column shifter. Get an exhaust guy to make you some head pipes that don't neck down and then run duals with a notched crossmember. It will run great with that cam even with a 600. Mine did! And I regretted cutting my inner for the super comps. Sure they fit but are they worth like the 10hp that will gain with that cam? Not really. I'd go with dougs if I had to.