waaaaay back when I had the Road Rocket, I held them in with a fairly chunky length of chain tied with a padlock, but one day a Pleeceman didn't like that idea

Short story, On this practice rebuild, I bought a (bargain at the time

) pair of stock headers (Made in England sticker-maybe Wassel

), and they kinda fitted ok, but the ends were squashed/tapered in and made it sloppy, so I stretched them out to fit ok (quite a process); all to no avail as after stuffing around I decided the bends/distance between are not compatible, so ended up cutting 30mm of the ends, which means the top radius starts too close to the flange so they also blow out....
possible remedy; I bought a small stainless steel turnbuckle from the boat-shop, to pull them together via the finnned flanges but haven't yet used it.
Was also thinking to use it in conjunction with some SS springs on the finned flanges.