Steering All Over the Place at 55mph+

Fellow A Body Folks... I have a 67 Barracuda and somewhere along the line it was converted to front discs. From installing new ball joints, tie rod ends, calipers, and discs it would seem that the upper control arm and upper/lower ball joints are from an E Body, the brake system from an M Body, and no idea about the spindle. The front wheelbase is slightly wider than the rear (rear is an 8-1/4 no clue what donated it). Not sure if it's an important variable but was originally a slant 6 car and now a 340. It is a power steering car. Ball joints, tie rod ends, and shocks have all been replaced and had it aligned (sorry the alignment report was lost along the way) at a chain tire shop. Tires are new 205/70 14's.

THE PROBLEM... since I've had it I've basically kept it under 55mph. I did notice if I do a sudden lane change it feels like I'm going to lose control. If you wobble the steering a wheel roughly a 1/4" either way it's pretty non-responsive. This last weekend I took it on an hour long cruise on the highway and was terrified at 65-70 mph. The best way to describe it was it felt like if I made any turn at at all (as simple a merging into another lane) I was going to lose control like it was going to twist (best words o can think to describe it). Even worse when hitting bumps or uneven pavement I thought that was the end for me and struggled to keep it in my lane.

In the forum's opinion can this simply be bad alignment? Can this be from the wider front wheelbase? I thought maybe I was the problem as this was the first non-rack and pinion car I've driven but others in my local Mopar club say that's not normal and are very comfortable doing 70mph. Any help or insight would be appreciated! Would love to go to shows a little further from home but no way I'm getting back on the freeway until this gets figured out. Thanks!

A lot of good info so far, but I can add a little.
When I first got my Dart it would darn near change lanes if I hit one of those reflectors in the road, and the feel of the car overall was exactly like you described.
Scary huh?

My problems were caused by badly worn lower control arm bushings.
Once I replaced those and aligned it with more caster it smoothed right out and got WAY more stable.
I commonly run 80-90 on the more open freeways now with total confidence.

Some already mentioned steering and suspension parts, but since I went through the exact same thing you described, I thought I would tell you.