Will these hold up to direct exhaust heat?

Stainless steel won't degrade from thermal cycling like nylon ties do. That said, these are meant to secure the heat wrap itself—not bear direct flame. Position them on the wrap surface, and they'll stay intact for the life of your wrap.

Do I need anything else to install heat wrap?

You'll need the heat wrap material itself. These ties do the fastening job. Start at one end, wrap the tie around the heated component and tape, cinch it snug, and trim the excess with a knife or cutters.

Can I reuse these if I remove the wrap later?

Once cinched tight, stainless zip ties don't unbind cleanly—you'll cut them off to remove the wrap. Plan to replace them if you take the wrap down and reinstall it elsewhere.