Which header gasket?

What would be a better gasket for my headers? Aluminum or copper? Also, What can I do to help prevent my bolts from backing out?

As far a the header gasket I would use what ever the manufacture of the header recommends. To keep the header bolts from backing out you can use loctite, but I wouldn't get any stronger than blue, incase you ever have to remove them.

The only reason im going with aluminum or copper is because im sick of the fiber ones blowing out.

Does anyone know what the gasket exhaust port size is? Is it the 1 1/2" or the 1 5/8" for an 1989 Chevy K1500 5.7L 4x4

I don't remember 'cause it's been a few years since I messed with them but the spare set I have in the garage measure 1 1/2" for my '88 K1500 350. I went with the reinforced gaskets (stamped steel between fiber) years ago and haven't had a blow out since. I don't remember if they're Mr Gasket or Felpro, but I got them locally.

The stock gasket is 1 1/2". The headers im running right now require me to use the 1 5/8". Just thought I would confirm it for those who may need it in the future.