Light Fixture Screws & Bracket Too Small for Fixture Box

Ceiling fans, when properly mounted, are put on specialized mounting/junction boxes that are especially strong, have a very secure mount for the fan and are hard screwed into a beam or stud.

What you have there looks like a pancake style fan box. See this example:

Every junction box has threaded holes for mounting fixtues such as your track lighting base. The studs in your picture appear to be the bolt holders for the fan. If they are stripped, it is not safe to hang things from them and oversized bolts are a risky work-around.

It may be that other screw holes in the bottom are properly threaded for standard fixture mounting. Check them using the bolts that came with the fixture. If they are not (and there are usually several unthreaded holes in junction boxes), you should probably replace the box with a new one with intact mounting tabs/holes.

The 2 3/4" and 3 1/2" sizes are both common. Many fixtures have screw slots for both.

A cross strap is a common way to adapt the larger size down to the smaller size. Screws to 3 1/2 box and 2 3/4 fixture mounts to strap bracket.

Some 2 3/4" boxes, particularly the metal styles, can allow a strap bracket to be mounted to the box and let the ends overhang the box so that screws on 3 1/2" centers can clear the outside of the box. In this case you can drill or open some holes under the bracket to allow the fixture mounting screws to go through the bracket and into the wall/ceiling.

You may want to also investigate the possibility to see if you can drill 2 3/4 mounting space holes in the fixture itself and forgo the use of any strap bracket itself.