That opens a whole new can of worms.

The vintage roadbikes all had 1" forks. Vintage MTBs had 1 1/8" forks.

You can, of course, get 1" threadless forks and headsets. And maybe 1" spacers. But, then the selection of 1" threadless stems is limited, so you'll probably end up with a 1 1/8" threadless stem and shims.

Anyway, if I was going to replace the fork anyway (steel ==> CF, building up forkless frame, etc), then I might consider converting from original threaded to threadless.

Otherwise, I'd probably stick with the threaded + adapters.

Or, just use the quills that have worked for ages. I put a winged handlebar on one bike necessitating a 2-bolt open faced stem, but somehow I accidentally found a 26.0 winged handlebar (which apparently is relatively rare now). And, there are a few quills that take the 26.0 bars with a 2 bolt face.