New Generation Slant six enthusiasts

The reason you you don't see the Dutra stuff and Clifford stuff isn't strictly because of the aging of the slant six. That stuff is just obsolete, period and a lot of people are basing the new toys on more current engines.

If someone released a DOHC aluminium head with a near bolt on EFI system, you'd see a boom in slant six enthusiasm. But for a lot of modern hotrodders, carbs are blase', cast iron is for tractors, and natural aspiration is what kills old people.

When you whittle down to the people that still use carbs, for example, there are still many but not as many as when that's all that there was. Whittle that population down further to those that like slant sixes, and then further to those with money..... and your pool is much much smaller.
Add in that people don't "do" castings so much anymore, that the cars that ran slant sixes are harder to find, and then the money thing again.

And the number of people that drove these cars new is rapidly approaching zero, the number of people that wax nostalgic for cars twenty years older than themselves (which seems to common to most car folks) are also beginning to thin out.

Out of those in the twenty year crowd (where I fall), many now have families and limited free money. The value of our seventies cars (and anything retro to the seventies) will bump again soon when our kids move out and money loosens.

This happened in the nineties with Harley and motorcycles in general (but Yamaha predicted it). Those that saw Easy Rider in the theatre had kids move out and had disposable income again. Now they're dying off and Harley sales are slowing down.