How much tire pressure up front? How about tread wear? Original wheel offset/scrub radius? I found the same thing when I modified my 68 FormulaS. I run about the same specs as you but with a modified oem PS box. I went back to a 14x6inch 0offset front wheel at 28psi. That helped a lot. Then I stuffed some wrenches in my pocket and went for a road test. I stopped every mile or two and adjusted my toe-in. Eventually I found a setting she liked.(dont remember what it ended up at).Finally I centered my Steering wheel. I also discovered that if the sector shaft endplay was set up too tight, it would produce similar symptoms.I also had to fix the bump-steer created by the large increase in pos caster. I cant see the 1/2* camber on my car. And I cant imagine how you could see caster with the wheel on the car. Hope this helps.