... when the nuts are off i can clean the threads up
Looks to me like the studs are just as bad as the nuts. If you split the latter, don't hold out too much hope of being left with studs that can be simply run with a die to clean up the threads ...
Same goes for the header clamps and collets, I'm afraid. The latter have corrosion-welded to the head from what I can see in the photo. Give it a go by all means but have one eye on ebay for a replacement set of headers.
Re the AIS tapping, if you don't have a tapered M8 in your Lidl set, be very very careful. The tap should cut easily, if it requires any effort to turn, back it out and rethink. Much better than than busting a tap in the stud as a few have discovered the hard way.
Good luck.