Is Mr Gasket #146 our only option these days for aftermarket/ported runners?

I don't know of any others.

Using a chassis punch, I enlarged the "stock" FelPro runner gaskets when I worked over the plenum and runners on mine. The slug-splitter type worked better than the standard old diamond point type, but possibly only because the die was a bit newer.

I'll go to kragen with my runners to see if they have enough material.

https://www.tpis.com/parts/view/22

Not sure who makes them, but TPIS sells them. They look identical to Mr. Gasket.