Calvin Elston and a few others have posted on Speedtalk forum that they will blacksmith a starter tube to the ports.

"If you are going to stretch it, have the tube align with the roof of the port and tack weld it, then stretch the tube to the floor. The match at the roof is the most important as that's where velocity is highest."
Posted in 340 street header questionsthread by 900HP Throttle's Perfomance

Last year when we built a set of headers for my 340 we learned about the variation in bolt hole placement relative to the ports (cast iron 340/360 heads). We suspect this is why off the shelf flanges we looked at have so much slop in the bolt holes. If you're real picky about getting your ports to line up nice, you may end up welding over and redrilling holes. We ended up having custom flanges made. I didn't check into what Ed had this time because the specifics of the project and also my builder prefered to work with St St flanges.

ps. Be warned that Ed can talk your ear off if you get him going. He does have very strong opinions... OTH he has some good ideas and his workmanship was very good. In particular I really liked the way he merged the primaries into the collectors. Below is a photo of the inside of the collector back from when Ed was making headers.