What type of bolts are suitable for a turbo exhaust housing?

Any zinc coating is a serious NO ( liquid metal -steel- embrittlement risk of stressed component like a bolt). The exhaust components can get pretty hot . Any standard steel bolting ( eg.chrome moly ) is going to start relaxing at 800 F, I think design temperatures are well above that. [relaxing comes at lower temperature than creeping]. I expect they are nickel alloy , there are a bunch so not worth listing them . And ,you only can get what is available in the automotive world, So you need to check with manufacturers or re-conditioners. Or , I can promise you that MP-35N would be great,Standard Pressed Steel , Jenkinstown PA , or Rolled Alloys . ...Just stick with manufacturer and reconditioners.