273 pistons?

The GM 4.3 liter V6 has a 4.00" bore, like a 350 but minus two cylinders. The 273 has a 3.625" bore. Maybe it was some other GM engine...

If you're looking for domed pistons like the commando motors had, Egge and Kanter are the only two I'm aware of. TRW used to make them, forged, but not anymore.

I think you have more options if you're looking for flat tops.

My engine thread details the issues I went through trying to dial in the compression ratio. What I found is that the Egge and Kanter pistons are not as tall as the OEM spec so you won't get the factory advertised compression ratio. That could be a good thing depending on what you're trying to do. I ended up at 9.6:1 with the Kanter pistons which I'm hoping will be good on pump gas.

The Kanter piston tops are as-cast so there is some variability from cylinder to cylinder. I believe Egge machines the tops of their pistons.

Good luck with your build.