TBH, I hadn't realised there was a temperature limitation on chemical metal. There might be a better equivalent, don't know. I'm not sure that the clamp itself would get to over 160 deg anyway. If you use Araldite, be sure to get the high temp version, again don't know what the limit is. I believe the chemical metal is pretty strong stuff. It's not like you have major movement on the pipes to strain it, probably a combination of corrosion/vibration caused it to go in the first place? Check that the headers are seated correctly with no strain on any particular clamp if you can. Also, if there's a lot of corrosion on the clamp, I'm not sure any bonding agent will hold anyway.