Can I weld or clamp this directly, or do I need to prep the finish first?

The black finish is already applied and can take welding or high-heat exposure. If you're clamping connections, the finish won't interfere — just tighten securely. For maximum durability at weld points, you can sand those areas bare before welding.

Does this pipe come with flanges or end fittings?

This is a bare U-bend pipe only. You'll need to source appropriate couplers, flanges, or adapters based on your specific plumbing or exhaust layout.

What's the wall thickness, and will it handle boost or high coolant pressure?

Wall thickness isn't specified in the product data — check with support or test-fit into your system before pressurizing. Aluminum handles moderate heat and pressure, but confirm specs match your application's demands.