Recently I bought a really nice used steel frame which came with a steel fork and a threaded headset. Originally I was planning to get a carbon fork and install a threadless headset, but then I decided to keep the steel fork.

However I have a contemporary aluminum handlebar (31.8"), and don't want to switch to an old style one and quill stem. So I ordered a quill stem converter:

https://www.profile-design.com/profil...converter.html

Is this a reliable solution? (ignoring the extra 100g weight Hey don't laugh, the steel frame itself is freaking light)

Anything I should be careful about? Also does anyone know how much torque should I use to tighten the quill converter?

It'll work fine and no worries at all. I've used one for thousands of miles. Note that you can improve the look some by putting a spacer below the bars so the converter won't be quite so obvious but this isn't necessary - purely for looks.

Since you're going into a steel steerer tube you shouldn't need to worry too much about over tightening as long as you don't use a 5 foot breaker bar on it If you're using a torque wrench, somewhere in the neighborhood of 19 Nm should be fine.

As said above, the adapter will work fine, and the only penalty is a bit of added weight. Since it's basically a quill stem,it's only function is to support the handlebat, and provide enough holding traction to steer the bike, which is very little. Rather than rely on a torque wrench, consider tightening it the old fashioned way, by degrees until the handlebar resists twisting when you hold the front wheel in your legs.

Decades ago, many applied the concept of "race tight" when tightening quill stems. Race tight was tight enough not to move under normal conditions, but still loose enough to twist in a crash. This helped reduce damage to front end parts in a crash. It isn't as practical with threadless systems since the stemm has to be tight enough to retain the headset adjustment.

Be sure of the length you need before you order one. I thought I'd like a long one, but that was a mistake.
Hey! Anyone need a 12" quill adapter? lol...