Do I need any hardware to install this, or does it bolt straight up?

The gasket itself installs between the header and manifold at the factory bolt holes — your existing header bolts will secure it. Bring the hardware you already have; this gasket is the seal piece only.

Can I reuse this gasket if I'm swapping headers later, or is it one-time only?

Exhaust gaskets should be replaced each time you break the seal. Once you bolt the header on and heat-cycle it, the gasket compresses and won't seal again — grab a fresh one for your next header swap.

Will this work with an aftermarket header, or only the factory manifold?

This gasket seals at the factory mounting surface. Any aftermarket header designed to bolt to your SI's stock connection points will work fine with this gasket — just confirm the header is SI-compatible first.