Do I need clamps or fasteners to secure this to my exhaust tubing?

The connector itself is a clamp that tightens around your tubing. You'll want band clamps or hose clamps to cinch it tight — verify they fit your 2.75" tubing diameter before ordering.

Can I weld this connector or does it have to stay bolted?

You can weld it. The clamp connection lets you dry-fit and test your exhaust routing first, then weld it permanently if you want a solid joint. Either method works.

Will this connector fit my 2.75" mandrel-bent tubing?

It's designed for 2.75" diameter tubing. Test-fit it on your specific bends first — tight radius bends or stepped tubing may affect how cleanly it seats.