It is “darting around” because it is following the crest of the lane or chasing bumps from “bump steer”. It is doing that because they set it up as if it were a stock VW sedan. Stock VW sedans sit much higher than our cars, which have all been lowered. When you lower the car, you have to add a lot of caster to get the steering geometry back to “normal”. Setting it to the specs for a 1970 VW sedan is fine except for just one thing:
They set your caster to 2.9° and 2.1° (L/R) and it should be set to somewhere in the range of 5° - 7° to eliminate the wandering and bump steer.
They probably won’t have the caster shims in stock to change your front end. The shims are placed behind the bottom torsion tube in the front end. I would suggest that you buy two (2) sets of shims AND a pair of longer mounting bolts, just in case you need them, and bring everything with you to the alignment place.