Just got my Demon moving under its own power - how much wander is normal before an alignment?

Nine times out of ten when a fresh front end darts like that it's the alignment, not the box. If the shop never touched it after you swapped arms and a new box, your toe is prolly all over the place, and toe is the one that makes a car feel nervous at low speed. Caster matters too. These old A-bodies like a decent chunk of positive caster for stability, and a lot of shops are scared to crank it in because the steering gets heavier. For autocross use you want it anyway, so tell them up front what the car is for. Also worth asking them to check bumpsteer, since aftermarket tubular upper arms sometimes sit the balljoint at a diffrent height than stock and that can add wander of its own.

Following this one since I'm in a similar boat, just with a 70s pickup instead of a Mopar. Mine got new tie rod ends and bushings last year and it wandered like crazy until I checked the tire pressures, turns out the shop had them way up in the 40s. Dropped them to spec and half the nervousness went away, so that might be worth a peek before you blame the box. Question for you though, since you set the preload yourself, did it feel notchy at center when you turned the wheel by hand with the front off the ground? Mine did and I never figured out if that was normal or not.

I'd push back a little on waving off the box so fast. When I put a rebuilt unit in my Dart I snugged the adjuster down chasing some play, and it made the steering stick right around center. Every little correction I made kept pulling past where I wanted, which felt a whole lot like dartiness til I backed it off and did it properly. A box that's too tight and an alignment that's never been set can both be wrong at the same time. I'd get the toe set somewhere cheap first just to take it off the list, then go back and recheck your adjuster before you spend real money at an alignment shop.

I'd bet that the extra weight of those tubular arms and strut rods is changing the car's natural roll center way more than you think. Even if the geometry is perfect, if the mass distribution is off, it'll feel jittery because it's handling weight shifts differently than the original setup. Did you also swap out the springs or just use new shocks on the old leaf spring setup? Sometimes those little changes in unsprung mass can totally mess with how responsive the steering feels, even if the alignment is dialed in tight.