67 Notchback disc brake/suspension questions

Too much we don't know. a new steering gear and or disc brakes may not have much effect on how it handles/stays in its lane. We don't know if its original 67 gearbox or if gearbox has ever been adjusted.
Someone may have simply went at the torsion bar adjustments to fit over sized shoes under there. Suspension geometry may be screwed up and causing bump steer. Offset upper arm bushings and alignment may cure some of it.
We don't know the condition of any steering related parts starting at the box coupling.
You haven't complained that the drum brakes wont stop it, so do whats needed to make it drive right first. Brakes can come later.