To make things flat like that I use a med honing stone. Cheap and works great. Need to re dress it hit it on a flat surface with course psa sandpaper on say glass.
When i took it off last night the last tube has a little twist in it when i loosened the bolts so i will prolly have to fix that, maybe cut the flange and straighten it out. Hoping to get away with the fat alum gaskets first.. Can't stand that 1 cyl header leak, reminds me of every 78 camaro everyone had when i was younger tick tick tick